now cos(pi/2 -x) is the positive version of sin(-x) and vice versa. Which means that if x=1 y=-0.844 if you graph them seperately you see they are opposite of each other which means that you will get 0.8444/-0.84444 or 1/-1

which means that the simplification is actually -1. This is something you need to memorize not something that will be shown in simplifying