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Trig Identities
• Simplifying Trig Expressions
• Proving Trig Identities
Pythagorean Identities:
Reciprocal Identities:
1
csc
sin
1
sec
cos
1
cot
tan
x
x
x
x
x
x
=
=
=
Tangent/Cotangent Identities:
x
x
x
x
x
x
sin
cos
cot
cos
sin
tan
=
=
Fundamental Trig Identities
2 2
sin cos 1x x+ = 2 2
tan 1 secx x+ =
2 2
1 cot cscx x+ =
Example 1
Simplify the trig expression: cos tan sint t t+
Solution: cos tan sint t t+ =
sin
cos sin
cos
t
t t
t
 
+  ÷
 
2 2
cos sin
cos
t t
t
+
=
1
cost
= sect=
2
sin
cos
cos
t
t
t
= +
Example 2
Simplify the expression: sin cot cosu u u+
Answer: cscu
Example 3
Simplify the expression:
cos cos
1 sin 1 sin
x x
x x
+ =
− +
cos cos
1 sin 1 sin
x x
x x
+
− +
cos (1 sin ) cos (1 sin )
(1 sin )(1 sin ) (1 sin )(1 sin )
x x x x
x x x x
+ −
+
− + + −
2 2
cos sin cos cos sin cos
1 sin 1 sin
x x x x x x
x x
+ −
= +
− −
2
2cos
1 sin
x
x
=
−
2
2cos
cos
x
x
=
2
2sec
cos
x
x
= =
Solution:
Tips for Proving Trig Identities
 Start with one side of the equation and manipulate it until it
equals the other side. (Try the more complicated side first!)
 Look for chances to use identities and/or algebraic
techniques (adding fractions, factoring, multiplying by a form
of “1”, etc.)
 If you get stuck, try re-writing everything in terms of the sine
and cosine.
 * Can also try working with each side of the equation
separately until you obtain the same expression.
Example 4
Prove the identity:
1 1
2tan sec
1 sin 1 sin
x x
x x
= −
− +
Example 5
Verify the identity:
2
1 cos tan
cos sec 1
θ θ
θ θ
+
=
−
1 cos
LHS =
cos
θ
θ
+ 1 cos
cos cos
θ
θ θ
= + sec 1θ= +
2
tan
RHS =
sec 1
θ
θ −
2
sec 1
sec 1
θ
θ
−
=
−
(sec 1)(sec 1)
sec 1
θ θ
θ
+ −
=
−
sec 1θ= +

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Trig identities

  • 1. Trig Identities • Simplifying Trig Expressions • Proving Trig Identities
  • 2. Pythagorean Identities: Reciprocal Identities: 1 csc sin 1 sec cos 1 cot tan x x x x x x = = = Tangent/Cotangent Identities: x x x x x x sin cos cot cos sin tan = = Fundamental Trig Identities 2 2 sin cos 1x x+ = 2 2 tan 1 secx x+ = 2 2 1 cot cscx x+ =
  • 3. Example 1 Simplify the trig expression: cos tan sint t t+ Solution: cos tan sint t t+ = sin cos sin cos t t t t   +  ÷   2 2 cos sin cos t t t + = 1 cost = sect= 2 sin cos cos t t t = +
  • 4. Example 2 Simplify the expression: sin cot cosu u u+ Answer: cscu
  • 5. Example 3 Simplify the expression: cos cos 1 sin 1 sin x x x x + = − + cos cos 1 sin 1 sin x x x x + − + cos (1 sin ) cos (1 sin ) (1 sin )(1 sin ) (1 sin )(1 sin ) x x x x x x x x + − + − + + − 2 2 cos sin cos cos sin cos 1 sin 1 sin x x x x x x x x + − = + − − 2 2cos 1 sin x x = − 2 2cos cos x x = 2 2sec cos x x = = Solution:
  • 6. Tips for Proving Trig Identities  Start with one side of the equation and manipulate it until it equals the other side. (Try the more complicated side first!)  Look for chances to use identities and/or algebraic techniques (adding fractions, factoring, multiplying by a form of “1”, etc.)  If you get stuck, try re-writing everything in terms of the sine and cosine.  * Can also try working with each side of the equation separately until you obtain the same expression.
  • 7. Example 4 Prove the identity: 1 1 2tan sec 1 sin 1 sin x x x x = − − +
  • 8. Example 5 Verify the identity: 2 1 cos tan cos sec 1 θ θ θ θ + = − 1 cos LHS = cos θ θ + 1 cos cos cos θ θ θ = + sec 1θ= + 2 tan RHS = sec 1 θ θ − 2 sec 1 sec 1 θ θ − = − (sec 1)(sec 1) sec 1 θ θ θ + − = − sec 1θ= +