Question

4cos^2x -2√2cosx -1 find x

Answer 1

If you take 4cos²x-2√2cosx-1 as a quadratic equation

and let x=cos x

then solve the equation 4x²-2√2x-1

then x=cosx=√2+√6 /4 or √2-√6 /4

Therefore x=cos^-1(√2+√6 /4) or cos^-1(√2-√6 /4)