Question

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

Answer 1

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