Question

The equation of a circle is (x + 3)2 + (y – 5)2 = 81. Determine the coordinates of the center of the circle and the length of the radius.

Answer 1

Answer:

The center of the circle is (-3 , 5) and the length of the radius is 9 units

Step-by-step explanation:

* Lets revise the standard form of the equation of the circle

- The center-radius form of the circle equation is in the format

 (x – h)² + (y – k)² = r², where the center is the point (h, k) and

 the radius is r.

- This form of the equation is helpful, because you can easily find

  the center and the radius.

* Now lets solve the problem

- The equation is (x + 3)² + (y - 5)² = 81

- By comparing the two equations

∵  (x – h)² + (y – k)² = r² and  (x + 3)² + (y - 5)² = 81

# x - h = x + 3

∴ -h = 3 ⇒ divide both sides by -1

∴ h = -3

# y - k = y - 5

∴ k = 5

# h and k are the coordinates of the center of the circle

∴ The center of the circle is (-3 , 5)

# r² = 81 ⇒ take √ for both sides

∴ r = 9

∴ The length of the radius = 9 units

* The center of the circle is (-3 , 5) and the length of the radius is 9 units