If cos x cos y = cos θ, prove that: sin^2 (x/2) cos^2 (y/2) +cos^2 (x/2) sin^2 (y/2) = sin^2 (θ/2)
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Originally Posted by yobacul If cos x cos y = cos θ, prove that: sin^2 (x/2) cos^2 (y/2) +cos^2 (x/2) sin^2 (y/2) = sin^2 (θ/2) Hint: Start with the formula . Apply that to all three angles in the formula , and see if you rearrange the result to get the desired conclusion.
I tried to .... I managed to get what I required on the RHS but I didn't manage to rearrange the rest to get the LHS.
Originally Posted by yobacul I managed to get what I required on the RHS but I didn't manage to rearrange the rest to get the LHS. Subtract 1 from both sides and divide by –2: Now write the left side of that as and you should be able to see how to complete the calculation from there.
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