If the angle is in standard position and P(x, y) is a point on the terminal side of , and r is the distance from the origin to P, then

x^2 + y^2 = r^2

Step-by-step explanation:

See image. "An angle in standard position" means the vertex (point part) of the angle is at the origin (0,0). And one side of the angle is glued onto the x-axis. The other side of the angle is free to rotate around the axis. That's the terminal side. Then there's a point P (x,y) on that side. See image. And r is labelled there. This set up makes a right triangle. So I put Pythagorean theorem as the answer here, but honestly if you are learning any right triangle theorems or trigonometry, you could use this set up. The leg that lays along the x-axis is x units long and the other leg is y units long. The hypotenuse is r units long.

step-by-step explanation: