The value of sinπ14sin3π14sin5π14sin7π14sin9π14sin11π14sin13π14 is equal to ___
A
164
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B
132
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C
116
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D
18
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Solution
The correct option is A164 sinπ14sin3π14sin5π14sin7π14sin9π14sin11π14sin13π14=(sinπ14sin3π14sin5π14)2=[cos(π2−π14)cos(π2−3π14)cos(π2−5π14)]2=[cos3π7cos2π7cosπ7]2=[cosπ7cos2π7cos4π7]2=(18sinπ7sin8π7)2=(sin(π+π7)8sinπ7)2=(−sinπ78sinπ7)2=(18)2=164