# Math Help - limits at infinity of trigs

1. ## limits at infinity of trigs

how does [cos(4x)-1] become [1- 2sin^2x -1] = 2sin^2x ?

which really means cos(4x) becomes 1-sin^2x

which trig identity does this use?

2. ## Re: limits at infinity of trigs

Originally Posted by kingsolomonsgrave

how does [cos(4x)-1] become [1- 2sin^2x -1] = 2sin^2x ?

which really means cos(4x) becomes 1-sin^2x

which trig identity does this use?
$\cos(2t)=1-2\sin^2(t)$ let $t=2x$.

3. ## Re: limits at infinity of trigs

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Thanks much!!!