# Thread: Double Angle Formulas

1. ## Double Angle Formulas

Hello, i am in a grade 12 advanced functions class (i guess it's pretty much a pre-calc class) and we're doing double angle formulas and i don't get it at all. Teacher isn't a calculus teacher so she can't explain it very well. Anywho, my question is:

Use a double angle formula to rewrite:
cos2(2x)

What i did was just times it out and got:
cos4x

But i don't know what i'm doing....

2. Originally Posted by CenturionMonkey
Hello, i am in a grade 12 advanced functions class (i guess it's pretty much a pre-calc class) and we're doing double angle formulas and i don't get it at all. Teacher isn't a calculus teacher so she can't explain it very well. Anywho, my question is:

Use a double angle formula to rewrite:
cos2(2x)

What i did was just times it out and got:
cos4x

But i don't know what i'm doing....

recall, the double angle formula for cosine says: $\cos 2A = \cos^2 A - \sin^2 A = 2 \cos^2 A - 1 = 1 - 2 \sin^2 A$
thus, $\cos 2(2x) = \cos^2 2x - \sin^2 2x = 2 \cos^2 2x - 1$
and you can go even further by expanding the $\cos 2x$