Convert the expression. cos 4a

Question

Convert the expression. cos 4a

Answer 1

cos4a = cos(2(2a)

=2cos^2(2a) -1

=2(2cos^2(a)-1)^2 -1

=2(4cos^4(a) -4cos^2(a) + 1) -1

=8cos^4(a) - 8cos^2(a) + 2 -1

= 8cos^4(a) - 8cos^2(a) + 1