Hi Sel,
2cos^2(x)+7cosx+3=0
The best method is you choose cos(x) = A then your equation will change to
2A^2 + 7A + 3 = 0 now it is much easier to solve. You know many method for this equation such as factor or quadratic formula
I use factor method
2A^2 + 7A + 3 = 0
( 2A +1) ( A + 3) = 0 ------> 2A + 1 = 0 both side minus 1 2A = -1 ----> both side divided by 2 A = -1/2
A + 3 = 0 both side add -3 A = -3
Now back to above that cos(x) = A -------> cos(x) = -1/2 and cos(x) = -3 unacceptable
cos(x) = -1/2 -------> x =2Π/3 and x = 4Π/3
I hope it is useful,
Minoo
