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Derive this identity from the sum and difference formulas for cosine: sin a sin b = (1...
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High School Teacher
We use the formula for sum and difference of two angles for cosine.
`cos(a - b) = cosacosb + sinasinb`
`cos(a + b) = cosacosb - sinasinb`
So, we will have:
`1/2[cosacosb + sinasinb - (cosacosb - sinasinb)] = 1/2[cosacosb + sinasinb - cosacosb + sinasinb]`
Combine like terms.
`1/2[2sinasinb] = sinasinb`
Hence, right hand side is equal to left hand side.
Posted by violy on April 21, 2013 at 12:40 AM (Answer #1)
Prostapheresis formula shows that:
`=-2sin a sin(-b)=2sina sin b`
`1/2(cos(a-b)-cos(a+b))=sin a sin b`
Posted by oldnick on April 21, 2013 at 1:03 AM (Answer #2)
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