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If cosalpha+cosbeta=a, sinalpha+sinbeta=...

If `cosalpha+cosbeta=a, sinalpha+sinbeta=b,` then `cos(alpha+beta)` is equal to (A) `(2ab)/(a^2+b^2)` (B) `(a^2+b^2)/(a^2-b^2)` (C) `(a^2-b^2)/(a^2+b^2)` (D) `(b^2-a^2)/(b^2+a^2)`

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