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Prove that: cos2alpha\ cos2beta+sin^2(al...

Prove that: `cos2alpha\ cos2beta+sin^2(alpha-beta)-sin^2(alpha+beta)=cos2(alpha+beta)` .

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LHS : `cos2alphacos2beta+sin^2(alpha−beta)−sin^2(alpha+beta) `
=`cos2alphacos2beta+sin(alpha−beta+alpha+beta)sin(alpha−beta−alpha−beta)`
=`cos2alphacos2beta−sin2alphasin2beta`
...
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