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

Question

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!