Trigonometry (cos)

**cloud5** · January 28th 2010, 06:13 AM

Show $cos \theta + cos (\theta + \alpha) + cos (\theta + 2\alpha) = (2 cos \alpha + 1) cos (\theta + \alpha)$

Answer:
$cos \theta + cos (\theta + \alpha) + cos (\theta + 2 \alpha)$
$=[cos (\theta + 2 \alpha) + cos \theta] + cos (\theta + \alpha)$
$=2 cos (\theta + \alpha) cos \alpha + cos (\theta + \alpha)$
$=(2 cos \alpha + 1)cos (\theta + \alpha)$

My question is what happen between this step?
$=2 cos (\theta + \alpha) cos \alpha + cos (\theta + \alpha)$
$=(2 cos \alpha + 1)cos (\theta + \alpha)$

**ANDS!** · January 28th 2010, 06:21 AM

$cos(\theta + \alpha)$ was factored out.

**Pulock2009** · January 28th 2010, 08:02 AM

please clarify your question. which step u didnot understand???

**cloud5** · January 28th 2010, 09:18 PM

Originally Posted by Pulock2009

please clarify your question. which step u didnot understand???

I wanna know the formula between $=2 cos (\theta + \alpha) cos \alpha + cos (\theta + \alpha)$
$=(2 cos \alpha + 1)cos (\theta + \alpha)$ how were they factorised?

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