The third one :) 8 is not greater than the absolute value of -9, which is 9
What is the surface area of the solid?
The sum of the area of all of the faces or surface that enclose the solid.
Cos² 2x = (1+cos 4x )/2
cos² 6x = (1+cos 12x)/2
subtract to get 1/2 (cos 4x - cos 12x )
= 1/2 (2 sin 4x sin 8x ) as cos A - cos B = 2 [ sin (A+B)/2 sin (A-B)/2 ]
so the answer