# How do you find the equation for the of the top half of a circle with radius 3?

##### 1 Answer

Mar 16, 2018

Solve for

#y = sqrt(9-x^2)#

#### Explanation:

The equation:

#x^2+y^2 = 3^2#

describes a circle of radius

If we attempt to solve for

Subtract

#y^2 = 9-x^2#

Take the square root of both sides to find:

#y = +-sqrt(9-x^2)#

Note that this gives us two values for

Since we are only interested in the upper half, we can select the non-negative square root to get the equation:

#y = sqrt(9-x^2)#

graph{sqrt(9-x^2) [-10, 10, -5, 5]}