Originally Posted by

**jeph** the question asks us to "eliminate the parameter to find a Cartesian equation of the curve"

x=sin(theta) , y=cos(theta) , theta is equal to or greater than 0 and equal to or less than pi

after you add and square the equation together you get x^2+y^2=1 because sin^2theta+cos^2theta=1. the answer says "Since theta is equal to or greater than 0 and equal to or less than pi, we have sin(theta) equal to or greater than 0, so x is equal to or greater than 0. **how did the domain change like that? why doesnt the domain just stay the same?**