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# Sum and Diffrences Formulas (trig)

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Im ripping my hair out with these stupid questions. How the heck do i find a cos A and cos B if only have sin A sin B?

From what i have here on my notes from my teacher, it shows different examples. If i had each of one i could do the the problem my just simply plugging it in the formula. So with these ones i tried to find them by doing this. I used this formula

cos ^ 2 = 1 - sin ^ 2        I would just plugin the both of the sin's? I know its wrong now that i need help,

Veteran  Mar 18, 2017
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### 2+0 Answers

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Using cos2 = 1 - sin2

cos2A = 1 - 5/25 = 20/25   so   cosA = 2√5/5

cos2B = 1 - 1/25 = 24/25   so   cosB = 2√6/5

cos(B-A) = cosBcosA + sinBsinA

sin(B-A) = sinBcosA - sinAcosB

Can you take it from here?

Alan  Mar 18, 2017
edited by Alan  Mar 19, 2017

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