i want to explain the solution in this question ..
I worked a red line on the steps that I did not understood it
On your unit circle, at an angle of 270 deg or 3pi/2, the coordinates of the intersection of the circle with the negative side of the y-axis is (0, -1). Cosine is defined as x/r, where x = x-coordinate of the circle and r = the radius.
Similarly, sine 270 is defined as y/r.
Print this image and study it: Unit Circle