This is the basic and exam oriented question from trigonometric identities in which we have given that (x/a sinθ – y/b cosθ) =1
and also (x/a cosθ + y/b sinθ) =1
we have to show that (x²/a² + y²/b²) =2
RS Aggarwal, Class 10, chapter 13B, question no 3
Hope it helps you