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cos (pi/7) cos ((2pi)/(7) ) cos ( (4pi )...

`cos (pi/7) cos ((2pi)/(7) ) cos ( (4pi )/(7) ) =`

A

`(-1)/(8)`

B

`1/8`

C

`-(3 sqrt3)/(8)`

D

1

Text Solution

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The correct Answer is:
A
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{:(,"Column-I",,"Column-II"),((A),cos 20^(0) + cos 80^(0) - sqrt(3) "cos" 50^(0),(p),-1),((B),cos 0^(0) + "cos" (pi)/(7) + "cos" (2pi)/(7) + "cos" (4pi)/(7) + "cos" (5pi)/(7) + "cos" (6 pi)/(7),(q),-(3)/(4)),((C),cos 20^(0) + cos 40^(0) + cos 60^(0) - 4 cos 10^(0) "cos" 20^(0)"cos" 30^(0),(r),1),((D),"cos" 20^(0) cos 100^(0) + cos 100^(0) "cos" 140^(0) - cos 140^(0) "cos" 200^(0),(s),0):}

cos. (pi)/(7) + cos. (2pi)/(7) + cos. (3pi)/(7) + cos. (4pi)/(7) + cos. (5pi)/(7) + cos. (6pi)/(7)=

Knowledge Check

  • sin "" (pi)/(7) sin "" (2pi)/(7) sin "" (4pi )/(7)=

    A
    `(sqrt7)/(8)`
    B
    `(sqrt7)/(4)`
    C
    `(sqrt7)/(2)`
    D
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  • The value of cos y cos ((pi)/(2) - x) - cos ((pi)/(2) - y) cos x + sin y cos ((pi)/(2) - x) + cos x sin ((pi)/(2) - y) is zero if

    A
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  • "cos" (2pi)/(7)+"cos" (4pi)/(7)+"cos"(6pi)/(7)=

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