# Math

posted by
**Momo**
.

Write a set of parametric equations in Quadrants I and IV for a semi-circle with a radius of 10.

I'm kind of new at parametrics, so this is what was given to me:

(x-h)^2 + (y-k)^2 = r^2

x^2 + y^2 = r^2

r = 10, y = rsint, x=rcost

So,

10cost + 10sint = 100

The answer is supposed to be

pi/2 < t < -pi/2

(actually greater than/less than or equal to)

My question is how do we get to the final solution, which is the pi/2 boundary. I know how to do the first part, I'm just not sure about the second...

Thank you!