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Question

cos8π7cos10π7cos12π7 is equal to

A
12
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B
12
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C
18
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D
14
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Solution

The correct option is B 18

cos8π7cos10π7cos12π7=cos8π7cos(2π4π7)cos(2π2π7)=cos8π7cos4π7cos2π7=2sin2π7cos2π7cos4π7cos8π72sin2π7=2sin4π7cos4π7cos8π74sin2π7=2sin8π7cos8π78sin2π7=sin16π78sin2π7=sin(2π+2π7)8sin2π7=sin2π78sin2π7=18


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