Question

If `(3-tan^(2)""(pi)/(7))/(1-tan^(2)""(pi)/(7))=kcos""(pi)/(7)` then the value of k is
A. 1
B. 2
C. 3
D. 4

Answer 1

Correct Answer - D
Let `theta=(pi)/(7)`
`rArr 3theta=pi-4theta`
`rArr sin3 theta=sin4theta`
`rArr 3sin theta-4 sin^(3)theta=4sinthetacos theta cos 2theta`
`rARr 3-4sin^(2)theta=4cos theta(2cos^(2)theta-1)`
`rArr 8cos^(3)theta-4cos theta=4cos^(2)theta-1`
`rArr 4costheta=(4cos^(2)theta-1)/(2cos^(2)theta-1)=(3-tan^(2)theta)/(1-tan^(2)theta)`