First, those are usually the definitions of sin, cos and tan....

But... opp is the length of the side opposite A, and hyp is the length of the side opposite the right angle so... opp/sin(A) = hyp/sin(90) since sin(90) = 1 sin(A) = opp/hyp

cos(A) = sin(90 - A) adj is the side opposite the angle (90 - A) so cos(A) = adj/hyp ... based on the previous result tan(A) = sin(A)/cos(a) = (opp/hyp)/(adj/hyp) from the previous results ==> tan(A) = opp/adj * hyp/hyp = opp/adj