(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Prove the following identity: [tex](1 + cos \theta)^2 + sin^2\theta = 2(1 + cos \theta)[/tex]

2. Relevant equations

[tex]cos^2 \theta + sin^2 \theta = 1[/tex]

3. The attempt at a solution

I've squared out the first bracket so that it becomes [tex]1 + cos^2 \theta[/tex] and multiplied out the second bracket so that it becomes [tex]2 + 2cos \theta[/tex]. With some rearrangement I get [tex]cos^2 \theta + sin^2 \theta = 1 + 2cos \theta[/tex]

I can't get rid of the 2cos, and I think it's because I'm doing something wrong when squaring out that first bracket. But I can't see what it is that I'm doing wrong, only that [tex]1 + cos^2 \theta[/tex] doesn't = [tex](1 + cos \theta)^2[/tex].

What am I doing wrong?

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# Homework Help: Proving this trigonometric identity

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