is equal to

12

If x = a cos θ and y = b sin θ, then b²x² + a²y² =

13

If x = a sec θ and y = b tan θ, then b²x²− a²y² =

15

2(sin⁶θ + cos⁶θ) − 3(sin⁴θ + cos⁴θ) is equal to

16

If a cos θ + b sin θ and a sin θ − b cos θ = 3, then a² + b² =