Question

cos 90 degrees?

Answer 1

cos (a) is called the abscissa of the point obtained by turning the point (1, 0) around the origin at an angle (a).

When you turn the point (1, 0) at an angle of 90 degrees, you will get the point (0, 1).

The abscissa of this point is zero, hence

сos (90°) = cos ($\pi$ /2) = 0.