Solve: sin 2x + cos x = 0

Prove that \(\tan ^{ -1 }{ \left( \frac { x }{ \sqrt { { { a }^{ 2 }-{ x }^{ 2 } } } } \right) } =\sin ^{ -1 }{ \left( \frac { x }{ a } \right) } \)

Prove that 4 cos 12° cos 48° cos 72° = cos 36°

Prove that cos 20° cos 40° cos 60° cos 80°

Prove that \(\frac{sin11AsinA+sin7Asin3A}{cos11AsinA+cos7Asin3A}=tan8A\)