Help with a trig to sum identity.

1. Apr 25, 2012

Gallani

I have been working on showing the equality between

N=0 to ∞ Ʃ cos(2nθ)(-1)^n/(2n)! = cos(cos(θ))cosh(sin(θ))

I started by using the standard series for cosine and putting cos(2nθ) in for the x term.

I did the same for cosh(sin(θ)). I manipulated the forms every way I could think but it never looked anything like the answer I seek.

2. Apr 25, 2012

scurty

You don't want to put $\cos{(2n \theta)}$ into the formula, you want $\cos{(\theta)}$. Maybe that's where you went wrong?

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