2017 · Q68
If x=a cosθ+b sinθ and y=a sinθ-b cosθ, then what is x²+y² equal to ?
Elementary MathematicsTrigonometry
- A2ab
- Ba+b
- Ca²+b²
- Da²-b²
Explanation
x² + y² = (a cosθ + b sinθ)² + (a sinθ − b cosθ)² = a² + b², the 2ab cross-terms cancelling: option C. Option D has the wrong sign.