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Express 2*sin54deg*sin66deg as a sum or difference of cosines.

1. cos12deg - cos120deg
2. cos12deg + cos120deg
3. sin120deg - sin12deg
4. sin120deg + sin12deg

Correct answer: cos12deg - cos120deg

Solution

Applying 2 sinX sinY = cos(X-Y) - cos(X+Y) with X=54, Y=66 gives cos12deg - cos120deg.

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