Cosy*(8cosy-7)/siny+1=0

[latex]cosy* frac{8cosy-7}{1-cos^2y} +1=0\ frac{8cos^2y-7}{1-cos^2y}+1=0\cosy=y\ frac{8y^2-7+1-y^2}{1-y^2}=0\ frac{7y^2-6}{(1+y)(1-y)}=0 \y_1=-1\y_2=1\y_3= sqrt{ frac{6}{7}} \x_1=pi+2 pi k;kZ \x_2=2 pi k;kZ[/latex]