2(sinx)^2-5cosx-4=0

Answer 1

change sin^2x to 1-cos^2x, then
2-2cos^2x-5cosx-4=0
let u= cosx
2-2u^2-5u-4=0
2u^2+5u+2=0
(2u+1)(U+2)=0
u=-1/2 u=-2
cosx=-1/2
x=330 deg disregard second answer, you are not dealing with hyperbolic trig functions yet (but there is a solution in the complex plane)