6 cos ^2 x+5 cos ( π/2 -x)=7
6 cos ^2 x+5sinx-7=0
6-6sin^2x+5sinx-7=0
-6sin^2 x+5sinx-1=0
6sin^2 x- 5 sinx+1=0
D= b^2 -4 ac
y1,2= 1/2 , 1/3
sin x=(-1)^n arcsin 1/2 πn , n ∈ Z ; x= (-1)^n arcsin 1/3 +πn
x= (-1)^n π/ 6 + πn,
