Question

2sin^2x-cosx-1=0

Answer 1

cosx= sqrt ( 1-Sin^2x)

Answer 2

Substitute instead of cos

Answer 3

sin^2 = 1- cos^2 so that your equation is 2-2cos^2 -cos -1 =0
\[cos^2(x)+cos(x)-1 =0 \]let cos(x) = t
it turns to t^2 +t-1=0
solve for t then plug back to solve for x

Answer 4

yeah, the other way ( sin = sqrt( 1- cos62x) ) works too