Question

Solve 4cos2A = 3cosA for 90º≤A≤180º.

I have absolutely no idea what I'm supposed to and my teacher won't help. :/

Answer 1

start with \(\cos(2x)=2\cos^2(x)-1\)

Answer 2

get
\[4(2\cos^2(x)-1)=3\cos(x)\] then
\[8\cos^2(x)-3\cos(x)-4=0\]

Answer 3

and then you get a really lousy quadratic equation to solve, so i am not sure what to do from there, other than solve it using the quadratic formula