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If alpha and beta are two real numbers s...

If `alpha` and `beta` are two real numbers such that `alpha- beta ne 2 n pi`, n is integer which satisfy the equation `a cos phi + b sin phi =c `, then prove that: `cos (alpha+ beta)= (a^2-b^2)/(a^2+b^2)`.

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MODERN PUBLICATION-TRIGNOMETRY-EXERCISE
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