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Questions  

The number of solutions ofr=15cosrx=5 in the interval 0,2π is

a
0
b
1
c
5
d
2

detailed solution

Correct option is D

∑r=15cosrx=5⇒cosx+cos2x+cos3x+cos4x+cos5x=5which is possible only when cosx=cos2x=cos3x=cos4x=cos5x=1 and is satisfied by x=0  and x=2π

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