23. II.1. Ća taĆÆp khaĆ» vi trong Rn. 23
taĆÆp 3 chieĆ u coĆ¹ bĆ“Ćø laĆø maĆ«t caĆ u āB cho bĆ“Ć»i: x2 + y2 + z2 = 1.
MeƤnh ƱeĆ . Cho M laĆø Ʊa taĆÆp khaĆ» vi k chieĆ u. Khi ƱoĆ¹:
(1) āM laĆø Ʊa taĆÆp khaĆ» vi k ā 1 chieĆ u khoĆ¢ng bĆ“Ćø, i.e. ā(āM) = ā .
(2) NeĆ”u x ā āM, thƬ TxāM laĆø khoĆ¢ng gian con k ā 1 chieĆ u cuĆ»a TxM.
ChĆ¶Ć¹ng minh: GoĆÆi i : Rkā1 ā Rk, i(u1, Ā· Ā· Ā· , ukā1) = (u1, Ā· Ā· Ā· , ukā1, 0). Khi ƱoĆ¹ deĆ£
thaĆ”y neĆ”u (Ļ, U) laĆø tham soĆ” hoaĆ¹ cuĆ»a M taĆÆi x vaĆø x ā āM, thƬ (Ļā¦i, iā1(U)) laĆø tham soĆ”
hoaĆ¹ cuĆ»a āM taĆÆi x. VĆ“Ć¹i tham soĆ” hoaĆ¹ ƱoĆ¹ x laĆø ƱieĆ„m trong cuĆ»a āM. VaƤy ā(āM) = ā .
HĆ“n nƶƵa TxāM laĆø khoĆ¢ng gian sinh bĆ“Ć»i D1Ļ(u), Ā· Ā· Ā· , Dkā1Ļ(u) neĆ¢n laĆø khoĆ¢ng gian con
k ā 1 chieĆ u cuĆ»a TxM.
1.7 ĆĆng duĆÆng vaĆøo baĆøi toaĆ¹n cƶĆÆc trĆ² ƱieĆ u kieƤn.
Cho F = (F1, Ā· Ā· Ā· , Fm) : V ā Rm, thuoƤc lĆ“Ć¹p C1 treĆ¢n taƤp mĆ“Ć» V ā Rn.
GoĆÆi M = {x ā V : F1(x) = Ā· Ā· Ā· = Fm(x) = 0}, vaĆø giaĆ» thieĆ”t rank F (x) = m, āx ā M.
Cho f : V ā R, thuoƤc lĆ“Ć¹p C1.
BaĆøi toaĆ¹n: TƬm cƶĆÆc trĆ² cuĆ»a haĆøm haĆÆn cheĆ” f|M . NoĆ¹i caĆ¹ch khaĆ¹c laĆø tƬm cƶĆÆc trĆ² cuĆ»a f vĆ“Ć¹i
ƱieĆ u kieƤn raĆøng buoƤc F1 = Ā· Ā· Ā· = Fm = 0.
NhaƤn xeĆ¹t. VƬ M laĆø Ʊa taĆÆp, neĆ¢n vĆ“Ć¹i moĆ£i a ā M toĆ n taĆÆi tham soĆ” hoaĆ¹ (Ļ, U) cuĆ»a M taĆÆi
a, vĆ“Ć¹i a = Ļ(b).
ĆieĆ u kieƤn caĆ n. NeĆ”u f ƱaĆÆt cƶĆÆc trĆ² vĆ“Ć¹i raĆøng buoƤc F1 = Ā· Ā· Ā· = Fm = 0, taĆÆi a, thƬ
grad f(a) ā„ TaM, i.e. toĆ n taĆÆi Ī»1, Ā· Ā· Ā· , Ī»m ā R, sao cho
grad f(a) = Ī»1grad F1(a) + Ā· Ā· Ā· + Ī»mgrad Fm(a)
ChĆ¶Ć¹ng minh: Theo nhaƤn xeĆ¹t treĆ¢n, roƵ raĆøng f|M ƱaĆÆt cƶĆÆc trĆ² taĆÆi a tƶƓng ƱƶƓng vĆ“Ć¹i f ā¦ Ļ
ƱaĆÆt cƶĆÆc trĆ² taĆÆi b.
Suy ra (f ā¦ Ļ) (b) = f (a)Ļ (b) = 0. VaƤy grad f(a), v = 0, āv ā ImĻ (b) = TaM,
i.e. grad f(a) ā„ TaM. Do rank (grad F1(a), Ā· Ā· Ā· , grad Fm(a)) = m = codimTaM,
neĆ¢n grad f(a) thuoƤc khoĆ¢ng gian sinh bĆ“Ć»i grad F1(a), Ā· Ā· Ā· , grad Fm(a).
PhƶƓng phaĆ¹p nhaĆ¢n tƶƻ hoaĆ¹ Lagrange. TƶĆø keĆ”t quĆ»a treĆ¢n, ƱeĆ„ tƬm ƱieĆ„m nghi ngĆ“Ćø cƶĆÆc trĆ²
cuĆ»a f vĆ“Ć¹i ƱieĆ u kieƤn F1 = Ā· Ā· Ā· = Fm = 0, ta laƤp haĆøm Lagrange
L(x, Ī») = f(x) ā Ī»1F1(x) ā Ā· Ā· Ā· ā Ī»mFm(x), x ā V, Ī» = (Ī»1, Ā· Ā· Ā· , Ī»m) ā Rm
NeĆ”u a laĆø cƶĆÆc trĆ² ƱieĆ u kieƤn, thƬ toĆ n taĆÆi Ī» ā Rm, sao cho (a, Ī») laĆø nghieƤm heƤ
ļ£±
ļ£“ļ£“ļ£“ļ£“ļ£“ļ£²
ļ£“ļ£“ļ£“ļ£“ļ£“ļ£³
āL
āx
(x, Ī») = 0
F1(x) = 0
...
Fm(x) = 0
VĆ duĆÆ. XeĆ¹t cƶĆÆc trĆ² f(x, y, z) = x + y + z, vĆ“Ć¹i ƱieĆ u kieƤn x2 + y2 = 1, x + z = 1.
TrĆ¶Ć“Ć¹c heĆ”t, ta thaĆ”y ƱieĆ u kieƤn raĆøng buoƤc xaĆ¹c Ć±Ć²nh moƤt Ʊa taĆÆp (Ellip E).
24. II.2 TĆch phaĆ¢n haĆøm soĆ” treĆ¢n Ʊa taĆÆp. 24
LaƤp haĆøm Lagrange L(x, y, z, Ī»1, Ī»2) = x + y + z ā Ī»1(x2 + y2 ā 1) ā Ī»2(x + z ā 1).
Giaƻi heƤ phƶƓng trƬnh
ļ£±
ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£²
ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£“ļ£³
āL
āx
= 1 ā 2Ī»1x āĪ»2 = 0
āL
āy
= 1 ā 2Ī»1y = 0
āL
āz
= 1 āĪ»2 = 0
x2 + y2 ā 1 = 0
x + z ā 1 = 0
Ta coĆ¹ caĆ¹c ƱieĆ„m nghi ngĆ“Ćø cƶĆÆc trĆ² laĆø (0, Ā±1, 1). Do taƤp ƱieĆ u kieƤn compact, neĆ¢n f phaĆ»i
ƱaĆÆt max, min treĆ¢n taƤp ƱoĆ¹. HĆ“n nƶƵa, caĆ¹c ƱieĆ„m cƶĆÆc trĆ² ƱoĆ¹ phaĆ»i laĆø moƤt trong caĆ¹c ƱieĆ„m
nghi ngĆ“Ćø cƶĆÆc trĆ². VaƤy
max f|E = max{f(0, 1, 1) = 1, f(0, ā1, 1) = 0} = f(0, 1, 1) = 1,
min f|E = min{f(0, 1, 1) = 1, f(0, ā1, 1) = 0} = f(0, ā1, 1) = 0
Trong trƶƓĆøng hĆ“ĆÆp taƤp ƱieĆ u kieƤn khoĆ¢ng compact, ta coĆ¹ theĆ„ sƶƻ duĆÆng keĆ”t quĆ»a sau:
ĆieĆ u kieƤn ƱuĆ». GiaĆ» sƶƻ f, F1, Ā· Ā· Ā· , Fm thuoƤc lĆ“Ć¹p C2, vaĆø
grad f(a) = Ī»1grad F1(a) + Ā· Ā· Ā· + Ī»mgrad Fm(a), i.e.
āL
āx
(a, Ī») = 0.
ĆaĆ«t HxL(x, a) laĆø Hessian cuĆ»a haĆøm Lagrange L theo bieĆ”n x. Khi ƱoĆ¹
NeĆ”u HxL(a, Ī»)|TaM xaĆ¹c Ć±Ć²nh dƶƓng, thƬ f|M ƱaĆÆt cƶĆÆc tieĆ„u taĆÆi a.
NeĆ”u HxL(a, Ī»)|TaM xaĆ¹c Ć±Ć²nh aĆ¢m, thƬ f|M ƱaĆÆt cƶĆÆc ƱaĆÆi taĆÆi a.
NeĆ”u HxL(a, Ī»)|TaM khoĆ¢ng xaĆ¹c Ć±Ć²nh daĆ”u, thƬ f|M khoĆ¢ng ƱaĆÆt cƶĆÆc trĆ² taĆÆi a.
ChĆ¶Ć¹ng minh: VĆ“Ć¹i caĆ¹c kyĆ¹ hieƤu Ć“Ć» phaĆ n treĆ¢n, baĆøi toaĆ¹n tƬm cƶĆÆc trĆ² cuĆ»a f|M tƶƓng ƱƶƓng baĆøi
toaĆ¹n tƬm cƶĆÆc trĆ² cuĆ»a fā¦Ļ. Do f (a)Ļ (b) = 0, tĆnh ƱaĆÆo haĆøm caĆ”p 2, ta coĆ¹ H(fā¦Ļ)(a)(h) =
Hf(a)(Ļ (b)h) (BaĆøi taƤp).
Do Fi ā¦ Ļ = 0, ta coĆ¹ H(Fi ā¦ Ļ) = 0 vaĆø theo tĆnh toaĆ¹n treĆ¢n H(Fi ā¦ Ļ)(b)(h) =
HFi(a)(Ļ (b)(h).
Suy ra HxL(a, Ī»)|TaM = H(f ā¦ Ļ)(b)|TaM .
TƶĆø ƱieĆ u kieƤn ƱuĆ» cuĆ»a baĆøi toaĆ¹n cƶĆÆc trĆ² Ć±Ć²a phƶƓng ta coĆ¹ keĆ”t quĆ»a. .
VĆ duĆÆ. Cho k ā N vaĆø a ā R. TƬm cƶĆÆc trĆ² f(x1, Ā· Ā· Ā· , xn) = xk
1 + Ā· Ā· Ā· + xk
n, vĆ“Ć¹i raĆøng
buoƤc x1 + Ā· Ā· Ā· + xn = an.
2. TĆCH PHAĆN HAĆM SOĆ TREĆN ĆA TAĆP
2.1 ĆoƤ daĆøi, dieƤn tĆch, theĆ„ tĆch trong R3. Trong R3, coĆ¹ trang bĆ² tĆch voĆ¢ hĆ¶Ć“Ć¹ng Euclid
Ā·, Ā· , neĆ¢n coĆ¹ khaĆ¹i nieƤm ƱoƤ daĆøi vaĆø vuoĆ¢ng goĆ¹c.
ĆoƤ daĆøi vector T = (xt, yt, zt): T = x2
t + y2
t + z2
t
25. II.2 TĆch phaĆ¢n haĆøm soĆ” treĆ¢n Ʊa taĆÆp. 25
DieƤn tĆch hƬnh bƬnh haĆønh taĆÆo bĆ“Ć»i u = (xu, yu, zu), v = (xv, yv, zv):
dt(u, v) = u vā„ = u Ć v
=
u 2 u, v
v, u v 2
1
2
= u 2 v 2 ā | u, v |2.
trong ƱoĆ¹ v = v + vā„ laĆø phaĆ¢n tĆch: v laĆø hƬnh chieĆ”u vuoĆ¢ng goĆ¹c v leĆ¢n u, vā„ ā„ u.
ChĆ¶Ć¹ng minh: Ta coĆ¹ v = Ī±u, vā„, u = 0. Suy ra
u, u u, v
v, u v, v
=
u, u u, v + u, vā„
v, u v, v + v, vā„
=
u, u Ī± u, u
v, u Ī± v, u
+
u, u 0
v, u vā„ 2
= u 2 vā„ 2
TƶĆø ƱoĆ¹ suy ra coĆ¢ng thĆ¶Ć¹c treĆ¢n
TheĆ„ tĆch khoĆ”i bƬnh haĆønh taĆÆo bĆ“Ć»i u, v, w ā R3:
tt(u, v, w) = dt(u, v) wā„
= | u Ć v, w | = | det(u, v, w)|
=
u, u u, v u, w
v, u v, v v, w
w, u w, v w, w
1
2
trong ƱoĆ¹ w = w +wā„ laĆø phaĆ¢n tĆch: w laĆø hƬnh chieĆ”u vuoĆ¢ng goĆ¹c w leĆ¢n maĆ«t phaĆŗng sinh
bƓƻi u, v.
Ā¢
Ā¢
Ā¢
Ā¢Ā¢w
E
u
ĀØĀØĀØB
v
Twā„ ĀØĀØĀØ
Ā¢
Ā¢
Ā¢
Ā¢Ā¢
ĀØĀØĀØĀ¢
Ā¢
Ā¢
Ā¢Ā¢
Ā¢
Ā¢
Ā¢
Ā¢Ā¢
ĀØĀØĀØ
ChĆ¶Ć¹ng minh: TƶƓng tƶĆÆ coĆ¢ng thĆ¶Ć¹c cho dieƤn tĆch. (BaĆøi taƤp)
2.2 TheĆ„ tĆch k chieĆ u trong Rn. Trong Rn coĆ¹ trang bĆ² tĆch voĆ¢ hĆ¶Ć“Ć¹ng Euclid. TheĆ„ tĆch
k chieĆ u cuĆ»a hƬnh bƬnh haĆønh taĆÆo bĆ“Ć»i v1, Ā· Ā· Ā· , vk ā Rn, ƱƶƓĆÆc Ć±Ć²nh nghĆ³a qui naĆÆp theo k:
V1(v1) = v1 , Vk(v1, Ā· Ā· Ā· , vk) = Vkā1(v1, Ā· Ā· Ā· , vkā1) vā„
k
trong ƱoĆ¹ vk = vk +vā„
k laĆø phaĆ¢n tĆch: vk laĆø hƬnh chieĆ”u vuoĆ¢ng goĆ¹c cuĆ»a vk leĆ¢n khoĆ¢ng gian
sinh bĆ“Ć»i v1, Ā· Ā· Ā· , vkā1.
CoĆ¢ng thĆ¶Ć¹c tĆnh. GoĆÆi G(v1, Ā· Ā· Ā· , vk) = ( vi, vj )1ā¤i,jā¤k laĆø ma traƤn Gramm. Khi ƱoĆ¹
Vk(v1, Ā· Ā· Ā· , vk) = det G(v1, Ā· Ā· Ā· , vk)
26. II.2 TĆch phaĆ¢n haĆøm soĆ” treĆ¢n Ʊa taĆÆp. 26
ChĆ¶Ć¹ng minh: TƶƓng tƶĆÆ coĆ¢ng thĆ¶Ć¹c cho dieƤn tĆch (BaĆøi taƤp).
2.3 PhaĆ n tƶƻ ƱoƤ daĆøi - ĆoƤ daĆøi ƱƶƓĆøng cong. Cho C ā R3 laĆø ƱƶƓĆøng cong cho bĆ“Ć»i tham
soĆ” hoaĆ¹
Ļ : I ā R3
, Ļ(t) = (x(t), y(t), z(t))
Ta caĆ n tĆnh ƱoƤ daĆøi l(C) cuĆ»a ƱƶƓĆøng cong.
PhaĆ¢n hoaĆÆch I thaĆønh caĆ¹c ƱoaĆÆn con Ii = [ti, ti + āti]. Khi ƱoĆ¹ l(C) = i l(Ļ(Ii)).
Khi āti beĆ¹, thƬ l(Ļ(Ii)) ā¼ l(Ļ (ti)āti) = Ļ (ti) āti.
ĆĆ²nh nghĆ³a phaĆ n tƶƻ ƱoƤ daĆøi : dl = Ļ (t) dt = x 2
t + y 2
t + z 2
t dt
ĆĆ²nh nghĆ³a ƱoƤ daĆøi cuĆ»a C:
l(C) =
C
dl =
I
x 2
t + y 2
t + z 2
t dt
2.4 PhaĆ n tƶƻ dieƤn tĆch - DieƤn tĆch maĆ«t. Cho S ā R3 laĆø maĆ«t cong cho bĆ“Ć»i tham soĆ”
hoaĆ¹
Ļ : U ā R3
, Ļ(u, v) = (x(u, v), y(u, v), z(u, v))
Ta caĆ n tĆnh dieƤn tĆch cuĆ»a maĆ«t S.
GƦa sƶƻ U coĆ¹ theĆ„ phaĆ¢n hoaĆÆch bĆ“Ć»i caĆ¹c hƬnh chƶƵ nhaƤt beĆ¹ Ui = [ui, ui+āui]Ć[vi, vi+āvi].
Khi ƱoĆ¹ dt(S) = i dt(Ļ(Ui)).
Khi āui, āvi beĆ¹, thƬ dt(Ļ(Ui)) ā¼ dt(D1Ļ(ui, vi)āui, D2Ļ(ui, vi)āvi).
ĆĆ²nh nghĆ³a phaĆ n tƶƻ dieƤn tĆch :
dS = dt(D1Ļ, D2Ļ)dudv = EG ā F2dudv,
trong ƱoĆ¹
E = D1Ļ 2 = xu
2
+ yu
2
+ zu
2
G = D2Ļ 2 = xv
2
+ yv
2
+ zv
2
F = D1Ļ, D2Ļ = xuxv + yuyv + zuzv
Khi ƱoĆ¹ Ć±Ć²nh nghĆ³a dieƤn tĆch cuĆ»a S :
dt(S) =
S
dS =
U
EG ā F2dudv
2.5 PhaĆ n tƶƻ theĆ„ tĆch - TheĆ„ tĆch hƬnh khoĆ”i. Cho H laĆø hƬnh khoĆ”i cho bĆ“Ć»i tham soĆ” hoaĆ¹
Ļ : A ā R3
, Ļ(u, v, w) = (x(u, v, w), y(u, v, w), z(u, v, w))
ĆeĆ„ tĆnh theĆ„ tĆch H, baĆØng laƤp luaƤn tƶƓng tƶĆÆ nhƶ caĆ¹c phaĆ n treĆ¢n, ta coĆ¹ caĆ¹c Ć±Ć²nh nghĆ³a:
PhaĆ n tƶƻ theĆ„ tĆch:
dV = tt(D1Ļ, D2Ļ, D3Ļ)dudvdw = | det JĻ|dudvdw
TheĆ„ tĆch H: V (H) = H dV = A | det JĻ|dudvdw.
BaĆ¢y giĆ“Ćø ta toĆ„ng quaĆ¹t hoaĆ¹ caĆ¹c khaĆ¹i nieƤm treĆ¢n.
27. II.2 TĆch phaĆ¢n haĆøm soĆ” treĆ¢n Ʊa taĆÆp. 27
2.6 PhaĆ n tƶƻ theĆ„ tĆch treĆ¢n Ʊa taĆÆp. Cho M ā Rn laĆø Ʊa taĆÆp khaĆ» vi k chieĆ u.
PhaĆ n tƶƻ theĆ„ tĆch treĆ¢n M laĆø aĆ¹nh xaĆÆ
dV : M x ā dV (x) = theĆ„ tĆch k chieĆ u haĆÆn cheĆ” treĆ¢n TxM.
GiaĆ»Ćø sƶƻ (Ļ, U) laĆø moƤt tham soĆ” hoaĆ¹ cuĆ»a M taĆÆi x = Ļ(u1, Ā· Ā· Ā· , uk). Khi ƱoĆ¹
dV (x)(D1Ļ(x)āu1, Ā· Ā· Ā· , DkĻ(x)āuk) = Vk(D1Ļ(x), Ā· Ā· Ā· , DkĻ(x))āu1 Ā· Ā· Ā· āuk
VaƤy neĆ”u ƱaĆ«t GĻ = ( DiĻ, DjĻ )1ā¤i,jā¤k, thƬ qua tham soĆ” hoĆ¹a
dV = det GĻ du1 Ā· Ā· Ā· duk
2.6 TĆch phaĆ¢n haĆøm treĆ¢n Ʊa taĆÆp. Cho f : M ā R laĆø haĆøm treĆ¢n Ʊa taĆÆp khaĆ» vi k chieĆ u.
Sau ƱaĆ¢y ta xaĆ¢y dƶĆÆng tĆch phaĆ¢n cuĆ»a f treĆ¢n M (coĆøn goĆÆi laĆø tĆch phaĆ¢n loaĆÆi 1)
M
fdV
NeĆ”u M = Ļ(U) vĆ“Ć¹i (Ļ, U) laĆø tham soĆ” hoĆ¹a, thƬ Ć±Ć²nh nghĆ³a
M
fdV =
U
f ā¦ Ļ det GĻ, trong ƱoĆ¹ GĻ = ( DiĻ, DjĻ )1ā¤i,jā¤k.
Khi k = 1 tĆch phaĆ¢n treĆ¢n goĆÆi laĆø tĆch phaĆ¢n ƱƶƓĆøng vaĆø kyĆ¹ hieƤu
M
fdl.
Khi k = 2 tĆch phaĆ¢n treĆ¢n goĆÆi laĆø tĆch phaĆ¢n maĆ«t vaĆø kyĆ¹ hieƤu
M
fdS.
TrƶƓĆøng hĆ“ĆÆp toĆ„ng quaĆ¹t, khi M cho bĆ“Ć»i nhieĆ u tham soĆ” hoĆ¹a, ngƶƓĆøi ta duĆøng kyƵ thuĆÆaĆ¢t phaĆ¢n
hoaĆÆch ƱƓn vĆ² sau ƱaĆ¢y ƱeĆ„ ādaĆ¹nā caĆ¹c tĆch phaĆ¢n treĆ¢n tƶĆøng tham soĆ” hoaĆ¹.
Cho O = {(Ļi, Ui) : i ā I} laĆø hoĆÆ caĆ¹c tham soĆ” hoaĆ¹ M. HoĆÆ Ī = {Īøi : i ā I} goĆÆi laĆø
phaĆ¢n hoaĆÆch ƱƓn vĆ² cuĆ»a M phuĆø hĆ“ĆÆp vĆ“Ć¹i hoĆÆ O neĆ”uu caĆ¹c ƱieĆ u sau thoĆ»a vĆ“Ć¹i moĆÆi i ā I:
(P1) Īøi : M ā [0, 1] lieĆ¢n tuĆÆc.
(P2) suppĪøi = {x ā M : Īø(x) = 0} laĆø taƤp compact.
(P3) suppĪøi ā Ļi(Ui).
(P4) MoĆÆi x ā M, toĆ n taĆÆi laĆ¢n caƤn V cuĆ»a x, sao cho chƦ coĆ¹ hƶƵu haĆÆn chƦ soĆ” i ā I
Īøi = 0 treĆ¢n V .
(P5) iāI Īøi(x) = 1, āx ā M.
TĆnh chaĆ”t (P4) goĆÆi laĆø tĆnh hƶƵu haĆÆn Ć±Ć²a phƶƓng cuĆ»a hoĆÆ {supp Īøi, i ā I}. Do tĆnh chaĆ”t
naĆøy toĆ„ng Ć“Ć» (P5) laĆø toĆ„ng hƶƵu haĆÆn vĆ“Ć¹i moĆÆi x.
ĆĆ²nh lyĆ¹. VĆ“Ć¹i moĆÆi hoĆÆ O caĆ¹c tham soĆ” hoaĆ¹ cuĆ»a Ʊa taĆÆp M, toĆ n taĆÆi hoĆÆ phaĆ¢n hoaĆÆch ƱƓn vĆ²
phuĆø hĆ“ĆÆp vĆ“Ć¹i O.
ChĆ¶Ć¹ng minh: GƦa sƶƻ M compact, k chieĆ u. VĆ“Ć¹i moĆÆi x ā M, toĆ n taĆÆi (Ļx, Ux) ā O laĆø
tham soĆ” hoaĆ¹ taĆÆi x. GoĆÆi Bx ā Ux laĆø moƤt hƬnh caĆ u taĆ¢n Ļā1
x (x). GƦa sƶƻ Bx = B(a, r).
HaĆøm gx : Rk ā R ƱƶƓĆÆc Ć±Ć²nh nghĆ³a nhƶ sau
gx(u) =
ļ£±
ļ£“ļ£²
ļ£“ļ£³
e
ā 1
r2ā uāa 2
, neĆ”u u ā a ā¤ r
0 , neĆ”u u ā a r.
45. IV. TĆch phaĆ¢n daĆÆng vi phaĆ¢n. 45
Cho S laĆø maĆ«t Ć±Ć²nh hĆ¶Ć“Ć¹ng trong R3. Ta caĆ n khaĆ¹i nieƤm tĆch phaĆ¢n cuĆ»a trƶƓĆøng vector F
qua maĆ«t S, hay laĆø tĆch phaĆ¢n cuĆ»a daĆÆng vi phaĆ¢n ĻF treĆ¢n S:
S
ĻF =
S
F1dx2 ā§ dx3 + F2dx3 ā§ dx1 + F3dx1 ā§ dx2
2.1 ĆĆ²nh nghĆ³a. Cho U laĆø taƤp mĆ“Ć» Rk, vaĆø Ļ ā ā¦k(U).
Khi ƱoĆ¹ Ļ = f(u)du1 ā§ Ā· Ā· Ā· ā§ duk. ĆĆ²nh nghĆ³a
U
Ļ =
U
f(u)du1 ā§ Ā· Ā· Ā· ā§ duk =
U
f(u)du1 Ā· Ā· Ā· duk.
neĆ”u tĆch phaĆ¢n veĆ” phaĆ»i toĆ n taĆÆi.
2.2 TĆch phaĆ¢n daĆÆng vi phaĆ¢n. Cho M laĆø Ʊa taĆÆp khaĆ» vi k chieĆ u Ć±Ć²nh hĆ¶Ć“Ć¹ng Āµ trong
Rn. Cho Ļ ā ā¦k(V ), vĆ“i V laĆø taƤp mĆ“Ć» chĆ¶Ć¹a M. Sau ƱaĆ¢y ta xaĆ¢y dƶĆÆng tĆch phaĆ¢n cuĆ»a
daĆÆng Ļ treĆ¢n M (coĆøn goĆÆi laĆø tĆch phaĆ¢n loaĆÆi 2)
M
Ļ
NeĆ”u M = Ļ(U) vĆ“Ć¹i (Ļ, U) laĆø moƤt tham soĆ” hoaĆ¹ xaĆ¹c Ć±Ć²nh hĆ¶Ć“Ć¹ng Āµ, thƬ Ć±Ć²nh nghĆ³a
M
Ļ =
U
Ļā
Ļ.
TrƶƓĆøng hĆ“ĆÆp toĆ„ng quaĆ¹t, khi M cho bĆ“Ć»i moƤt hoĆÆ tham soĆ” hoaĆ¹ O = {(Ļi, Ui) : i ā I} xaĆ¹c
Ć±Ć²nh hĆ¶Ć“Ć¹ng Āµ, ta duĆøng kyƵ thuaƤt phaĆ¢n hoaĆÆch ƱƓn vĆ². GoĆÆi Ī = {Īøi : i ā I} laĆø phaĆ¢n
hoaĆÆch ƱƓn vĆ² cuĆ»a M phuĆø hĆ“ĆÆp vĆ“Ć¹i O. ĆĆ²nh nghĆ³a
M
Ļ =
iāI Ļi(Ui)
ĪøiĻ =
iāI Ui
Ļā
i (ĪøiĻ) ,
vĆ“Ć¹i giaĆ» thieĆ”t veĆ” phaĆ»i toĆ n taĆÆi. ChaĆŗng haĆÆn khi M compact vaĆø Ļ lieĆ¢n tuĆÆc.
Khi k = 1, tĆch phaĆ¢n coĆ¹ daĆÆng
M i
Fidxi, vaĆø goĆÆi laĆø tĆch phaĆ¢n ƱƶƓĆøng.
Khi k = 2, tĆch phaĆ¢n coĆ¹ daĆÆng
M ij
Fijdxi ā§ dxj, vaĆø goĆÆi laĆø tĆch phaĆ¢n maĆ«t.
NhaƤn xeĆ¹t. ĆĆ²nh nghĆ³a treĆ¢n khoĆ¢ng phuĆÆ thuoƤc caĆ¹ch choĆÆn hoĆÆ tham soĆ” xaĆ¹c Ć±Ć²nh hĆ¶Ć“Ć¹ng Āµ
vaĆø phaĆ¢n hoaĆÆch ƱƓn vĆ².
ChĆ¶Ć¹ng minh: Khi hai tham soĆ” hoĆ¹a (Ļ, U) vaĆø (Ļ, W), cuĆøng xaĆ¹c Ć±Ć²nh hĆ¶Ć“Ć¹n Āµ, ta coĆ¹
Ļ = Ļ ā¦ h, vĆ“Ć¹i h laĆø vi phoĆ¢i coĆ¹ det Jh 0. NeĆ”u ĻāĻ = f(u)du1 ā§ Ā· Ā· Ā· ā§ duk, thƬ
hā(f(u)du1 ā§ Ā· Ā· Ā· ā§ duk) = hāĻāĻ = (Ļ ā¦ h)āĻ = ĻāĻ.
Theo coĆ¢ng thĆ¶Ć¹c ƱoĆ„i bieĆ”n, ta coĆ¹
U
Ļā
Ļ =
U
f =
W
f ā¦ ā¦h det Jh =
W
hā
(f(u)du1 ā§ Ā· Ā· Ā· ā§ duk) =
W
Ļā
Ļ.
VaƤy Ć±Ć²nh nghĆ³a khoĆ¢ng phuĆÆ thuoƤc tham soĆ” hoaĆ¹ xaĆ¹c Ć±Ć²nh cuĆøng hĆ¶Ć“Ć¹ng.
NeĆ”u Ī = {Īøj : j ā J} laĆø moƤt phaĆ¢n hoaĆÆch ƱƓn vĆ² khaĆ¹c cuĆ»a M. Khi ƱoĆ¹
j M
ĪøjĻ =
j M
(
i
Īøi)ĪøjĻ =
i,j M
ĪøiĪøjĻ =
i,j M
ĪøjĪøiĻ =
i M
(
j
Īøj)ĪøiĻ
i M
ĪøiĻ.
46. IV. TĆch phaĆ¢n daĆÆng vi phaĆ¢n. 46
VaƤy Ć±Ć²nh nghĆ³a cuƵng khoĆ¢ng phuĆÆ thuoƤc phaĆ¢n hoaĆÆch ƱƓn vĆ².
2.3 TĆnh chaĆ”t. Cho M laĆø Ʊa taĆÆp k chieĆ u Ć±Ć²nh hĆ¶Ć“Ć¹ng Āµ trong taƤp mĆ“Ć» V . Khi ƱoĆ¹
(1)
M
: ā¦k
(V ) ā R laĆø tuyeĆ”n tĆnh.
(2)
M
Ļ = ā
āM
Ļ , vĆ“Ć¹i kyĆ¹ hieƤu āM ƱeĆ„ chƦ M Ć±Ć²nh hĆ¶Ć“Ć¹ng āĀµ.
ChĆ¶Ć¹ng minh: (1) suy tƶĆø tĆnh tuyeĆ”n tĆnh cuĆ»a Ui
vaĆø Ļā
i .
(2) XeĆ¹t pheĆ¹p ƱoĆ„i bieĆ”n h(u1, Ā· Ā· Ā· , uk) = (āu1, Ā· Ā· Ā· , uk). Khi ƱoĆ¹ det h = ā1. NeĆ”u
(Ļ, U) laĆø tham soĆ” hoaĆ¹ xaĆ¹c Ć±Ć²nh hĆ¶Ć“Ć¹ng Āµ, thƬ (Ļ ā¦ h, hā1(U)) laĆø tham soĆ” hoaĆ¹ xaĆ¹c Ć±Ć²nh
hĆ¶Ć“Ć¹ngāĀµ. TƶĆø ƱoĆ¹ suy ra vĆ“Ć¹i moĆÆi phaĆ¢n hoaĆÆch ƱƓn vĆ² Ī phuĆø hĆ“ĆÆp vĆ“Ć¹i hoĆÆ tham soĆ” hoaĆ¹,
ta coĆ¹
āM
Ļ =
ĪøāĪ hā1(U)
(Ļ ā¦ h)ā
ĪøĻ =
ĪøāĪ
(ā
U
Ļā
ĪøĻ) = ā
M
Ļ.
VĆ duĆÆ.
a) Cho C laĆø ƱƶƓĆøng cong trĆ“n, cho bĆ“Ć»i tham soĆ” hoĆ¹a Ļ : I ā Rn, Ć±Ć²nh hĆ¶Ć“Ć¹ng theo chieĆ u
taĆŖng cuĆ»a tham soĆ”. Khi ƱoĆ¹
C i
Fidxi =
I i
Fi ā¦ ĻdĻi =
I
(
i
Fi ā¦ Ļ(t)Ļi(t))dt.
ChaĆŗng haĆÆn, neĆ”u ƱƶƓĆøng troĆøn ƱƓn vĆ² Ć±Ć²nh hĆ¶Ć“Ć¹ng ngƶƓĆÆc chieĆ u kim ƱoĆ ng hoĆ , thƬ
x2+y2=1
ydx ā xdy
x2 + y2
=
2Ļ
0
sin td(cos t) ā cos td(sin t)
cos2 t + sin2
t
= ā
2Ļ
0
dt = ā2Ļ.
b) Cho S laĆø maĆ«t caĆ u ƱƓn vĆ² Ć±Ć²nh hĆ¶Ć“Ć¹ng phaĆ¹p trong, thƬ vĆ“Ć¹i tham soĆ” hoaĆ¹ xaĆ¹c Ć±Ć²nh
hĆ¶Ć“Ć¹ng tƶƓng Ć¶Ć¹ng, ta coĆ¹
S
xdy ā§ dz =
[0,2Ļ]Ć[0,Ļ]
cos Ļ sin Īød(sin Ļ sin Īø) ā§ d(cos Īø)
=
[0,2Ļ]Ć[0,Ļ]
cos Ļ sin Īø(cos Ļ sin ĪødĻ + sin Ļ cos ĪødĪø) ā§ d(ā sin ĪødĪø)
=
[0,2Ļ]Ć[0,Ļ]
ā cos2
Ļ sin3
ĪødĻ ā§ dĪø =?
2.4 Quan heƤ giƶƵa tĆch phaĆ¢n loaĆÆi 1 vaĆø loaĆÆi 2.
Cho F = (P, Q, R) laĆø trƶƓĆøng vector lĆ“Ć¹p C1 treĆ¢n moƤt taƤp mĆ“Ć» V ā R3.
(1) Cho C ā V laĆø ƱƶƓĆøng cong kĆn, Ć±Ć²nh hĆ¶Ć“Ć¹ng bĆ“Ć»i trƶƓĆøng vector tieĆ”p xuĆ¹c ƱƓn vĆ²
T = (cos Ī±, cos Ī², cos Ī³). Khi ƱoĆ¹
C
Pdx + Qdy + Rdz =
C
F, T dl =
C
(P cos Ī± + Q cos Ī² + R cos Ī³)dl.
(2) Cho S ā V laĆø maĆ«t trĆ“n, Ć±Ć²nh hĆ¶Ć“Ć¹ng bĆ“Ć»i trƶƓĆøng phaĆ¹p vector ƱƓn vĆ² N =
(cos Ī±, cos Ī², cos Ī³). Khi ƱoĆ¹
S
Pdyā§dz+Qdzā§dx+Rdxā§dy =
S
F, N dS =
S
(P cos Ī±+Q cos Ī²+R cos Ī³)dS.
49. IV.3 CoĆ¢ng thƶc Stokes 49
trong R, Ļ(x) = xdx.
3.2 CaĆ¹c coĆ¢ng thĆ¶Ć¹c coĆ„ ƱieĆ„n. Sau ƱaĆ¢y laĆø caĆ¹c heƤ quĆ»a cuĆ»a Ć±Ć²nh lyĆ¹ treĆ¢n:
CoĆ¢ng thĆ¶Ć¹c Newton-Leibniz. Cho V laĆø taƤp mĆ“Ć» trong Rn, F : V ā R thuoƤc lĆ“Ć¹p C1
vaĆø Ļ : [a, b] ā V laĆø tham soĆ” hoaĆ¹ ƱƶƓĆøng cong trĆ“n. Khi ƱoĆ¹
Ļ([a,b])
dF = F(Ļ(b)) ā F(Ļ(a)).
CoĆ¢ng thĆ¶Ć¹c Green. Cho D ā R2 laĆø mieĆ n compact, coĆ¹ bĆ“Ćø C = āD Ć±Ć²nh hĆ¶Ć“Ć¹ng ngƶƓĆÆc
chieĆ u kim ƱoĆ ng hoĆ . Cho P, Q laĆø caĆ¹c haĆøm lĆ“Ć¹p C1 treĆ¢n taƤp mĆ“Ć» chĆ¶Ć¹a D. Khi ƱoĆ¹
D
(
āQ
āx
ā
āP
āy
)dxdy =
C
Pdx + Qdy.
CoĆ¢ng thĆ¶Ć¹c Stokes coĆ„ ƱieĆ„n. Cho S ā R3 laĆø maĆ«t cong trĆ“n Ć±Ć²nh hĆ¶Ć“Ć¹ng phaĆ¹p N, coĆ¹ bĆ“Ćø
āS = C laĆø ƱƶƓĆøng cong kĆn Ć±Ć²nh hĆ¶Ć“Ć¹ng sao cho mieĆ n phĆa traĆ¹i. Cho P, Q, R caĆ¹c haĆøm
lĆ“Ć¹p C1 treĆ¢n moƤt taƤp mĆ“Ć» chĆ¶Ć¹a S. Khi ƱoĆ¹
S
(
āQ
āx
ā
āP
āy
)dxā§dy+(
āR
āy
ā
āQ
āz
)dyā§dz+(
āP
āz
ā
āR
āx
)dzā§dx =
C
Pdx+Qdy+Rdz.
CoĆ¢ng thĆ¶Ć¹c Gauss-Ostrogradski. Cho V ā R3 laĆø mieĆ n compact, coĆ¹ bĆ“ĆøƵ āV = S laĆø maĆ«t
trĆ“n Ć±Ć²nh hĆ¶Ć“Ć¹ng phaĆ¹p ngoaĆøi. Cho P, Q, R laĆø caĆ¹c haĆøm lĆ“Ć¹p C1 treĆ¢n moƤt mieĆ n mĆ“Ć» chĆ¶Ć¹a
V . Khi ƱoĆ¹
V
(
āP
āx
+
āQ
āy
+
āR
āz
)dxdydz =
S
Pdy ā§ dz + Qdz ā§ dx + Rdx ā§ dy.
VĆ duĆÆ.
a) DieƤn tĆch mieĆ n D giĆ“Ć¹i haĆÆn bĆ“Ć»i ƱƶƓĆøng cong kĆn C trong R2:
D
dxdy =
C
xdy = ā
C
ydx =
1
2 C
(xdy ā ydx).
b) TheĆ„ tĆch mieĆ n V giĆ“Ć¹i haĆÆn bĆ“Ć»i maĆ«t cong kĆn S trong R3:
V
dxdydz =
S
xdy ā§ dz =
S
ydz ā§ dx =
S
zdx ā§ dy
=
1
3
(
S
xdy ā§ dz +
S
ydz ā§ dx +
S
zdx ā§ dy)
3.3 MeƤnh ƱeĆ . GƦa sƶƻ U laĆø taƤp mĆ“Ć», co ruĆ¹t ƱƶƓĆÆc trong Rn. Cho Ļ =
n
i=1
aidxi ā ā¦1
(U).
Khi ƱoĆ¹ caĆ¹c ƱieĆ u sau tƶƓng ƱƶƓng:
(1) Ļ laĆø khĆ“Ć¹p, i.e. toĆ n taĆÆi f ā C1(U), sao cho df = Ļ.
(2) Ļ laĆø ƱoĆ¹ng, i.e. dĻ = 0.
51. IV.3 CoĆ¢ng thƶc Stokes 51
Khi ƱoĆ¹ bieĆ„u ƱoĆ sau giao hoaĆ¹n
Cā(U)
grad
ā X (U)
rot
ā X (U)
div
ā Cā(U)
ā id ā h1 ā h2 ā h3
ā¦0(U)
d
ā ā¦1(U)
d
ā ā¦2(U)
d
ā ā¦3(U)
nghĆ³a laĆø ta coĆ¹: h1 ā¦ grad = d ā¦ id, h2 ā¦ rot = d ā¦ h1, h3 ā¦ div = d ā¦ h2.
ChĆ¶Ć¹ng minh: Xem nhƶ baĆøi taƤp
HeƤ quĆ»a. TƶĆø d ā¦ d = 0, suy ra rot ā¦ grad = 0, div ā¦ rot = 0.
3.5 CoĆ¢ng thĆ¶Ć¹c Stokes cho tĆch phaĆ¢n loaĆÆi 1. Cho F laĆø moƤt trƶƓĆøng vector khaĆ» vi trong
R3.
(1) GiaĆ» sƶƻ S laĆø maĆ«t cong compact trong R3, Ć±Ć²nh hĆ¶Ć“Ć¹ng bĆ“Ć»i trƶƓĆøng vector phaĆ¹p ƱƓn vĆ²
N, coĆ¹ bĆ“Ćø āS = C laĆø ƱƶƓĆøng cong Ć±Ć²nh hĆ¶Ć“Ć¹ng caĆ»m sinh bĆ“Ć»i trƶƓĆøng vector tieĆ”p xuĆ¹c ƱƓn
vĆ² T sao cho mieĆ n S naĆØm phĆa traĆ¹i. Khi ƱoĆ¹
C
F, T dl =
S
rot F, N dS.
(2) GiaĆ» sƶƻ V laĆø mieĆ n giĆ“Ć¹i noƤi trong R3 coĆ¹ bĆ“Ćø āV = S laĆø maĆ«t cong Ć±Ć²nh hĆ¶Ć“Ć¹ng bĆ“Ć»i
trƶƓĆøng vector phaĆ¹p ƱƓn vĆ² N hĆ¶Ć“Ć¹ng ra phĆa ngoaĆøi. Khi ƱoĆ¹
S
F, N dS =
V
div FdV.
ChĆ¶Ć¹ng minh: Suy tƶĆø coĆ¢ng thĆ¶Ć¹c Stokes vaĆø moĆ”i quan heƤ giƶƵa tĆch phaĆ¢n loaĆÆi 1 vaĆø loaĆÆi 2.
56. BaĆøi taƤp 56
II. TĆch phaĆ¢n haĆøm treĆ¢n Ʊa taĆÆp
1. Cho f : Rn
ā Rm. ChĆ¶Ć¹ng minh f khaĆ» vi lĆ“Ć¹p Cp khi vaĆø chƦ khi ƱoĆ thĆ² f laĆø Ʊa
taĆÆp khaĆ» vi lĆ“Ć¹p Cp trong Rn Ć Rm.
2. Cho F : Rn ā Rm laĆø aĆ¹nh xaĆÆ khaĆ» vi. GoĆÆi M laĆø taƤp con cuĆ»a Rm cho bĆ“Ć»i heƤ
phƶƓng trƬnh F(x) = 0. ChĆ¶Ć¹ng minh neĆ”u rank F (x) = m vĆ“Ć¹i moĆÆi x ā M, thƬ
M laĆø Ʊa taĆÆp khaĆ» vi n ā m chieĆ u.
3. Cho Ī± : (a, b) ā R2 laĆø tham soĆ” hoaĆ¹ ƱƶƓĆøng cong trĆ“n, Ī±(t) = (x(t), y(t)) vaĆø
y(t) 0. ChĆ¶Ć¹ng minh maĆ«t troĆøn xoay cho bĆ“Ć»i tham soĆ” hoaĆ¹:
Ļ(t, Īø) = (x(t), y(t) cos Īø, y(t) sin Īø), (t, Īø) ā (a, b) Ć (0, 2Ļ),
laĆø moƤt Ʊa taĆÆp khaĆ» vi trong R3.
ChĆ¶Ć¹ng minh caĆ¹c ƱƶƓĆøng cong toĆÆa ƱoƤ laĆø vuoĆ¢ng goĆ¹c vĆ“Ć¹i nhau. TƬm vector phaĆ¹p vaĆø
maĆ«t phaĆŗng tieĆ”p xuĆ¹c.
AĆp duĆÆng: haƵy tham soĆ” hoaĆ¹ maĆ«t truĆÆ, caĆ u, xuyeĆ”n.
4. Cho Ī± : (a, b) ā R2 laĆø tham soĆ” hoaĆ¹ moƤt ƱƶƓĆøng cong trĆ“n vaĆø p = (p1, p2, p3) ā R3
vĆ“Ć¹i p3 = 0. ChĆ¶Ć¹ng minh maĆ«t noĆ¹n cho bĆ“Ć»i tham soĆ” hoaĆ¹:
Ļ(t, s) = (1 ā s)p + s(Ī±(t), 0), (t, s) ā (a, b) Ć (0, 1),
laĆø Ʊa taĆÆp khaĆ» vi trong R3. XaĆ¹c Ć±Ć²nh caĆ¹c ƱƶƓĆøng cong toĆÆa ƱoƤ, vector phaĆ¹p, maĆ«t
phaĆŗng tieĆ”p xuĆ¹c.
5. KieĆ„m tra caĆ¹c taƤp cho bĆ“Ć»i caĆ¹c phƶƓng trƬnh hay tham soĆ” sau laĆø Ʊa taĆÆp khoĆ¢ng.
Trong R2: a) x = a(1 ā sin t), y = a(1 ā cos t) b) x = t2, y = t3.
Trong R3: a) x = a cos t, y = a sin t, z = bt (a, b laĆø caĆ¹ haĆØng soĆ” dƶƓng)
b) x =
ā
2 cos 2t, y = sin 2t, z = sin 2t
c)
x2
a2
+
y2
b2
+
z2
c2
= 1 d)
x2
a2
+
y2
b2
ā
z2
c2
= Ā±1 e)
x2
a2
+
y2
b2
ā z = 1
f) x = (b + a cos Īø) cos Ļ, y = (b + a cos Īø) sin Ļ, z = a sin Īø
g) x2 + y2 = z2
y2 = ax
h) x2 + y2 = a2
x + y + z = 0
TƬm phƶƓng trƬnh ƱƶƓĆøng thaĆŗng hay maĆ«t phaĆŗng tieĆ”p xuĆ¹c cho caĆ¹c Ʊa taĆÆp treĆ¢n.
6. KieĆ„m tra caĆ¹c phƶƓng trƬnh vaĆø baĆ”t phƶƓng trƬnh sau xaĆ¹c Ć±Ć²nh Ʊa taĆÆp coĆ¹ bĆ“Ćø trong
R3:
a) x2 + y2 + z2 = 1, z ā„ 0 b) x2 + y2 ā¤ a2, x + y + z = 0
c) x2 + y2 + z2 ā¤ a2, x + z = 0 d) z2 ā¤ y2 + x2, z = a.
7. ChĆ¶Ć¹ng minh trong R3, maĆ«t caĆ u x2 + y2 + z2 = a2 khoĆ¢ng theĆ„ cho bĆ“Ć»i moƤt tham
soĆ” hoaĆ¹, nhƶng coĆ¹ theĆ„ cho bĆ“Ć»i hai tham soĆ” hoaĆ¹.
8. XaĆ¹c Ć±Ć²nh phƶƓng trƬnh cuĆ»a khoĆ¢ng gian tieĆ”p xuĆ¹c taĆÆi (x0, f(x0)) cho Ʊa taĆÆp Ć“Ć» baĆøi
taƤp 1.
57. BaĆøi taƤp 57
9. PhaĆ¹c hoĆÆa caĆ¹c maĆ«t, roĆ i xaĆ¹c Ć±Ć²nh caĆ¹c ƱƶƓĆøng cong toĆÆa ƱoƤ, vector phaĆ¹p, khoĆ¢ng gian
tieĆ”p xuĆ¹c cuĆ»a caĆ¹c maĆ«t cho bĆ“Ć»i tham soĆ” hoaĆ¹::
a) Ļ(t, Īø) = (t cos Īø, t sin Īø, Īø). (maĆ«t Helicoid).
b) Ļ(t, Īø) = ((1 + t cos Īø
2 ) cos Īø, (1 + t cos Īø
2 ) sin Īø, t sin Īø
2), |t|
1
4
, Īø ā (0, 2Ļ).
(laĆ¹ M ĀØobius)
10. XeĆ¹t Ʊa taĆÆp M cho Ć“Ć» baĆøi taƤp 2. GoĆÆi F = (F1, Ā· Ā· Ā· , Fm).
a) ChĆ¶Ć¹ng minh khi ƱoĆ¹ khoĆ¢ng gian tieĆ”p xuĆ¹c cuĆ»a M laĆø
TxM = ker F (x) = {v ā Rn
: grad F1(x), v = Ā· Ā· Ā· = grad Fm(x), v = 0 }.
b) Cho f : Rn ā R. ChĆ¶Ć¹ng minh neĆ”u f ƱaĆÆt cƶĆÆc trĆ² vĆ“Ć¹i ƱieĆ u kieƤn x ā M = {x :
g(x) = 0} taĆÆi a, thƬ toĆ n taĆÆi Ī»1, Ā· Ā· Ā· , Ī»m ā R, sao cho
grad f(a) = Ī»1grad F1(a) + Ā· Ā· Ā· + Ī»mgrad Fm(a).
11. XeĆ¹t cƶĆÆc trĆ² haĆøm:
a) f(x, y) = ax + by, vĆ“Ć¹i ƱieĆ u kieƤn x2 + y2 = 1.
b) f(x, y, z) = x ā 2y + 2z, vĆ“Ć¹i ƱieĆ u kieƤn x2 + y2 + z2 = 1.
c) f(x, y, z) = x2 + y2 + z2, vĆ“Ć¹i ƱieĆ u kieƤn
x2
a2
+
y2
b2
+
z2
c2
= 1 (a b c 0).
d) f(x, y, z) = xyz, vĆ“Ć¹i caĆ¹c ƱieĆ u kieƤn: x2 + y2 + z2 = 1, x + y + z = 0.
e) f(x, y, z) = x + y + z, vĆ“Ć¹i caĆ¹c ƱieĆ u kieƤn: x2 + y2 = 2, x + z = 1.
12. XeĆ¹t cƶĆÆc trĆ² caĆ¹c haĆøm:
a) f(x, y, z) = x2 + y2 + z2, vĆ“Ć¹i ƱieĆ u kieƤn x2 + y2 ā 2 ā¤ z ā¤ 0.
b) f(x, y, z) = x2 + 2y2 + 3z2, vĆ“Ć¹i ƱieĆ u kieƤn x2 + y2 + z2 ā¤ 100.
13. TƬm theĆ„ tĆch lĆ“Ć¹n nhaĆ”t cuĆ»a caĆ¹c hƬnh hoƤp chƶƵ nhaƤt vĆ“Ć¹i ƱieĆ u kieƤn dieƤn tĆch maĆ«t laĆø
10m2.
14. ChĆ¶Ć¹ng minh trung bƬnh hƬnh hoĆÆc khoĆ¢ng lĆ“Ć¹n hĆ“n trung bƬnh soĆ” hoĆÆc, i.e.
(a1 Ā· Ā· Ā· an)
1
n ā¤
1
n
(a1 + Ā· Ā· Ā· + an), (a1, Ā· Ā· Ā· , an 0)
15. ChĆ¶Ć¹ng minh baĆ”t ƱaĆŗng thĆ¶Ć¹c
x + y
2
n
ā¤
xn + yn
2
, (x, y 0, n ā N).
(HD: XeĆ¹t cƶĆÆc trĆ² f(x, y) =
xn + yn
2
, vĆ“Ć¹i ƱieĆ u kieƤn x + y = s).
16. ChĆ¶Ć¹ng minh baĆ”t ƱaĆŗng thĆ¶Ć¹c H ĀØolder:
n
i=1
aixi ā¤ (
n
i=1
ap
i )
1
p (
n
i=1
xq
i )
1
q , neƔu xi, ai 0,
1
p
+
1
q
= 1 (p, q 0).
61. BaĆøi taƤp 61
5. Cho Ī± : [a, b] ā R2 {0} laĆø moƤt tuyeĆ”n. GiaĆ» sƶƻ
Ī±(t) = (x(t), y(t)) = (r(t) cos Īø(t), r(t) sin Īø(t)) vĆ“Ć¹i x, y, r, Īø laĆø caĆ¹c haĆøm khaĆ» vi.
a) ChĆ¶Ć¹ng minh Īø (t) =
āy(t)x (t) + x(t)y (t)
x2(t) + y2(t)
.
b) XeĆ¹t Ļ =
āydx + xdy
x2 + y2
. ChĆ¶Ć¹ng minh Ļ ƱoĆ¹ng nhƶng khoĆ¢ng khĆ“Ć¹p.
c) ĆĆ²nh nghĆ³a chƦ soĆ” voĆøng quay cuĆ»a Ī± quanh 0:
I(Ī±, 0) =
1
2Ļ Ī±
Ļ =
b
a
āy(t)x (t) + x(t)y (t)
x2(t) + y2(t)
dt
TĆnh chƦ soĆ” treĆ¢n khi Ī±(t) = (a cos kt, a sin kt), t ā [0, 2Ļ].
6. TĆnh
C
(y2
ā z2
)dx + (z2
ā x2
)dy + (x2
ā y2
)dz,
trong ƱoĆ¹ C laĆø chu vi tam giaĆ¹c caĆ u: x2 + y2 + z2 = 1, x, y, z ā„ 0, Ć±Ć²nh hĆ¶Ć“Ć¹ng
caĆ»m sinh hĆ¶Ć“Ć¹ng phaĆ¹p ngoaĆøi maĆ«t caĆ u..
7. Cho S laĆø ƱoĆ thĆ² haĆøm z = x2 + y2 + 1, (x, y) ā (0, 1)2. HaƵy xaĆ¹c Ć±Ć²nh moƤt hĆ¶Ć“Ć¹ng
cho S roĆ i tĆnh
S
ydy ā§ dz + xzdx ā§ dz
8. TĆnh tĆch phaĆ¢n Ʊo goĆ¹c khoĆ”i cuĆ»a maĆ«t S ƱoĆ”i vĆ“Ć¹i goĆ”c 0:
S
xdy ā§ dz + ydz ā§ dx + zdx ā§ dy
(x2 + y2 + z2)3/2
trong trƶƓĆøng hĆ“ĆÆp S laĆø: a) MaĆ«t caĆ u. b) Nƶƻa maĆ«t caĆ u. c) MoƤt phaĆ n taĆ¹m maĆ«t caĆ u.
9. Trong R3, cho S : 4x2 + y2 + 4z2 = 4, y ā„ 0.
a) PhaĆ¹c hoĆÆa S vaĆø āS.
b) Tham soĆ” hoaĆ¹ S bĆ“Ć»i Ļ(u, v) = (u, 2(1 ā u2 ā v2)
1
2 , v). XaĆ¹c Ć±Ć²nh hĆ¶Ć“Ć¹ng cho
bĆ“Ć»i tham soĆ” Ļ.
c) Cho Ļ = ydx + 3xdz. TĆnh
āS
Ļ vaĆø
S
dĻ.
10. AĆp duĆÆng coĆ¢ng thĆ¶Ć¹c Green, tĆnh: I =
C
xy2
dy ā x2
ydx, vĆ“Ć¹i C : x2 + y2 = a2
Ć±Ć²nh hĆ¶Ć“Ć¹ng ngƶƓĆÆc chieĆ u kim ƱoĆ ng hoĆ .
11. AĆp duĆÆng coĆ¢ng thĆ¶Ć¹c Green, tĆnh dieƤn tĆch hƬnh giĆ“Ć¹i haĆÆn bĆ“Ć»i ƱƶƓĆøng cong trong R2
cho bƓƻi phƶƓng trƬnh
x
a
n
+
y
b
n
= 1. (a, b, n 0).
12. Cho I =
C
xdx + ydy + zdz,
vĆ“Ć¹i C laĆø ƱƶƓĆøng troĆøn: x2 + y2 + z2 = a2, x + y + z = 0, vĆ“Ć¹i Ć±Ć²nh hĆ¶Ć“Ć¹ng tƶĆÆ choĆÆn.
a) TĆnh trƶĆÆc tieĆ”p I. b) DuĆøng coĆ¢ng thĆ¶Ć¹c Stokes tĆnh I.