cos 5x = -cos x

cos 5x+cos x = 0

We know

Here

Now from the above equation it would be,

2cos 3x cos 2x=0

cos 3x cos 2x=0

cos 3x=0 or cos 2x=0

for cos3x=0

for cos2x=0

so the values of the x less than equal to 90° are