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Question:

The value of cos210° - cos10°cos50° + cos250° is?

3/2

3/2(1+cos20°)

3/4+cos20°

3/4

Solution:

1/2(2cos210° - 2cos10°cos50° + 2cos250°) => 1/2(1 + cos20° - (cos60° + cos40°) + 1 + cos100°) => 1/2(3/2 + cos20° + 2sin70°sin(-30°)) => 1/2(3/2 + cos20° - sin70°) => 1/2(3/2 + sin70° - sin70°) => 3/4
Ans (2).