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If `A+B+C=3pi/2`. Then `cos 2A +cos 2B+cos2C` is equal toA. `1-4cos A cos B cos C`B. `4 sin A sin B sinC`C. `1+2cos A cos B cos C`D. `1-4sin A sin B sinC` |
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Answer» Correct Answer - D `cos 2A+cos2B+cos 2C` `=2cos(A+B)cos(A-B)+cos2C` `=2cos(A+B)cos(A-B)+cos2C` `=2cos((3pi)/(2)-C)cos(A-B)+cos2C` `=2cos((3pi)/(2)-C)cos(A-B)+cos2C` `=-2sin C cos (A-B)+1-2sin^(2)C` `=1-2sinC cos(A-B)+sinC)` `=1-2sinC(cos(A-B)+sin[3pi//2-(A+B)]]` `=1-2sinC[cos(A-B)-cos(A+B)]` `=1-4sinA sin B sin C`. |
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