A certain circle's area is x square units, and its circumference is y units. The value of x+y is 80pi. What is the radius of the circle, in units?

Guest Feb 22, 2017

#1**0 **

I will take a crack at it:

Area =Pi x r^2

Circumference =Pi x 2r, since:

x = Area

y = Circumference

[Pi r^2] + [Pi 2r] =80Pi

Solve for r:

2 π r + π r^2 = 80 π

Divide both sides by π:

r^2 + 2 r = 80

Add 1 to both sides:

r^2 + 2 r + 1 = 81

Write the left hand side as a square:

(r + 1)^2 = 81

Take the square root of both sides:

r + 1 = 9 or r + 1 = -9

Subtract 1 from both sides:

r = 8 or r + 1 = -9

Subtract 1 from both sides:

__ Answer: |r = 8__ or r = -10(Discard)

Guest Feb 22, 2017