Hello! Are you familiar with the Unit Circle? The unit circle is like the lifeblood of trigonometry. Look up an image right now. Learn about it. If you look at the circle E and notice the location of the line EM as it intercepts the circle E, and then you compare it to the unit circle, you will notice that the line intercepts at the location ~90 degrees on the unit circle (or pi/2 in radians).
That might not make sense right away, but study the unit circle and you'll get it!!
Hope that helps!