Question

4sina*cosa*(cos²a-sin²a)​

Answer 1

$\underbrace {4\, sina\cdot cosa}_{2\cdot sin2a}\cdot (\underbrace {cos^2a-sin^2a}_{cos2a})=2\cdot sin2a\cdot cos2a=sin4a\\\\\\\star \; \; 2\cdot sinx\cdot cosx=sin2x\; \; \star \\\\\star \; \; cos^2x-sin^2x=cos2x\; \; \star$