a) \(sin^4x-cos^4x=\left(sin^2x-cos^2x\right)\left(sin^2x+cos^2x\right)\)
\(=\left(sin^2x-1+sin^2x\right).1=2sin^2x-1\)
b) \(\dfrac{1}{sin^2x}+\dfrac{1}{cos^2x}=\dfrac{cos^2x+sin^2x}{sin^2x}+\dfrac{cos^2x+sin^2x}{cos^2x}\)
\(=1+\dfrac{cos^2x}{sin^2x}+1+\dfrac{sin^2x}{cos^2x}=2+tan^2x+cot^2x\)