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CONIC
SECTIONS
-CIRCLES-
CIRCLES
• Geometric: the result of a
cone and a plane
intersecting each other
• Algebraic: set of points on a
plane that are equidistant
from a fixed point on the
same plane (center/origin)
x
y
Radius (r)
Center (h,k)
- The distance from the center of the circle (h,k) to any point on the
circle (x,y).
RADIUS
(x,y)
Equation of the Circle
• The center-radius or standard form:
(𝑥 − ℎ)2+(𝑦 − 𝑘)2 = 𝑟2
where (h,k) is the center and r is the radius
• Derived from the Pythagorean Theorem
𝑎2 + 𝑏2 = 𝑟2 𝑎
𝑏
Find the center and the radius for each of the
following circles.
1. (𝑥 − 4)2
+(𝑦 − 6)2
= 25
2. (𝑥 − 3)2
+(𝑦 + 1)2
= 16
3. (𝑥 + 5)2
+ 𝑦 − 2 = 15
4. 𝑥2
+ (𝑦 + 7)2
= 18
ans. center is (4,6) & r = 5
ans. center is (3,-1) & r = 4
ans. center is (-5,2) & r = 15
ans. center is (0,-7) & r = 3 2
Write the equation of the circle given the
center and the radius.
1. C (2,-7) & r = 3
(𝑥 − 2)2+(𝑦 + 7)2= 9
2. C (-3, -3) & r = 10
(𝑥 + 3)2+(𝑥 + 3)2= 10
Write the equation of a circle whose diameter
is the line segment joining A(-3,-4) and B(4,3).
• What must you find first?
The center and the radius.
• How can you find the center?
The center is the midpoint of the segment.
(½ , - ½ )
• How can you find the radius?
The radius is the distance from the center to a point on the circle.
Use the distance formula.
r = 7
1
2
2 2
1 1 49
2 2 2
x y
   
      
   
Find the equation of the circle.
𝑥2
+ 𝑦2
+ 6𝑥 − 4𝑦 − 12 = 0
Hint: You must complete the square
2 2
6 ___ 4 ___ 12x x y y     
2 2
6 9 4 4 12 9 4x x y y       
2 2
( 3) ( 2) 25x y   

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Circles

  • 2. CIRCLES • Geometric: the result of a cone and a plane intersecting each other • Algebraic: set of points on a plane that are equidistant from a fixed point on the same plane (center/origin)
  • 3. x y Radius (r) Center (h,k) - The distance from the center of the circle (h,k) to any point on the circle (x,y). RADIUS (x,y)
  • 4. Equation of the Circle • The center-radius or standard form: (𝑥 − ℎ)2+(𝑦 − 𝑘)2 = 𝑟2 where (h,k) is the center and r is the radius • Derived from the Pythagorean Theorem 𝑎2 + 𝑏2 = 𝑟2 𝑎 𝑏
  • 5. Find the center and the radius for each of the following circles. 1. (𝑥 − 4)2 +(𝑦 − 6)2 = 25 2. (𝑥 − 3)2 +(𝑦 + 1)2 = 16 3. (𝑥 + 5)2 + 𝑦 − 2 = 15 4. 𝑥2 + (𝑦 + 7)2 = 18 ans. center is (4,6) & r = 5 ans. center is (3,-1) & r = 4 ans. center is (-5,2) & r = 15 ans. center is (0,-7) & r = 3 2
  • 6. Write the equation of the circle given the center and the radius. 1. C (2,-7) & r = 3 (𝑥 − 2)2+(𝑦 + 7)2= 9 2. C (-3, -3) & r = 10 (𝑥 + 3)2+(𝑥 + 3)2= 10
  • 7. Write the equation of a circle whose diameter is the line segment joining A(-3,-4) and B(4,3). • What must you find first? The center and the radius. • How can you find the center? The center is the midpoint of the segment. (½ , - ½ ) • How can you find the radius? The radius is the distance from the center to a point on the circle. Use the distance formula. r = 7 1 2 2 2 1 1 49 2 2 2 x y               
  • 8. Find the equation of the circle. 𝑥2 + 𝑦2 + 6𝑥 − 4𝑦 − 12 = 0 Hint: You must complete the square 2 2 6 ___ 4 ___ 12x x y y      2 2 6 9 4 4 12 9 4x x y y        2 2 ( 3) ( 2) 25x y   