Question

cos2a = ???

Answer 1

I am sorry I dont know.

Answer 2

:)

Answer 3

cos(2a)="any value between -1 and +1"

Answer 4

\[cos (2a)=\cos^2(a)-\sin^2(a)\]

Answer 5

pther identities are

cos(2a)=2cos^2(a)− 1
and
cos(2a)=1−2sin^2(a)

Answer 6

Oh, is about identities?\[\cos(2a)=\frac{ e^{i2a}+e^{-i2a} }{ 2 }\]is a good one