If B is 90 degrees then the side AC is the hypotenuse of the right-angle triangle. So as per Pythagoras theorem:AC2 = AB2 + BC2If we divide each term in the above equation by AC2 we get:AC2/AC2 = AB2/AC2 + BC2/AC2I.e (AC/AC)2 = (AB/AC)2 + (BC/AC)2So, 1 = Cos2θ + Sin2θ, where θ is the angle opposite to side BC.