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Express 2*sin(50 deg)*sin(70 deg) as a sum or difference of cosines.

1. cos(20 deg) - cos(120 deg)
2. cos(20 deg) + cos(120 deg)
3. sin(120 deg) - sin(20 deg)
4. sin(120 deg) + sin(20 deg)

Correct answer: cos(20 deg) - cos(120 deg)

Solution

Applying 2 sinA sinB = cos(A-B) - cos(A+B) with A=50, B=70 gives cos(20 deg) - cos(120 deg).

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