1.

If in ` A B C ,A=pi/7,B=(2pi)/7,C=(4pi)/7`then `a^2+b^2+c^2`must be`R^2`(b) `3R^2`(c) `4R^2`(d)`7R^2`A. `R^(2)`B. `3R^(2)`C. `4R^(2)`D. `7R^(2)`

Answer» Correct Answer - D
`a^(2) + b^(2) + c^(2) = 4R^(2) (sin^(2) A + sin^(2) B + sin^(2)C)`
`= 2R^(2) (3 - (cos 2A + cos 2B + cos 2C))`
Now, `cos 2A + cos 2B + cos 2C`
`= cos.(2pi)/(7) + cos.(4pi)/(7) + cos.(8pi)/(7)`
`= cos.(2pi)/(7) + cos.(4pi)/(7) + cos.(6pi)/(7)`
`= (sin.(3pi)/(7))/(sin.(pi)/(7)) cos.(4pi)/(7)`
`= (-2 sin.(3pi)/(7) cos.(3pi)/(7))/(2 sin.(pi)/(7))`
`= (-sin.(6pi)/(7))/(2sin.(pi)/(7)) = (-1)/(2)`
`:. a^(2) +b^(2) + c^(2) = 2R^(2) (3-((-1)/(2))) = 7R^(2)`


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