This is the basic and conceptual question from trigonometric ratios in which we are to prove that (cos^2θ – sin^2 θ) = 41/841, if in a ∆ABC, in which ∠C = 90°, ∠ABC = θ°, BC = 21 units, AB = 29 units.
RS Aggarwal, Class 10, Chapter 10, question no 26.
Hope it helps you.