Log of Sine

Algebra Level 4

If e ( sin 2 x + sin 4 x + sin 6 x + . . . . . . . . . . ) ln 2 { e }^{ (\sin ^{ 2 }{ x } +\sin ^{ 4 }{ x } +\sin ^{ 6 }{ x } +..........\infty )\ln { 2 } } for 0 < x < π 2 0<x<\frac { \pi }{ 2 } , satisfies the equation x 2 9 x + 8 = 0 { x }^{ 2 }-9x+8=0 The value of 1 1 + tan x \frac { 1 }{ 1 +\tan { x } } is of the form a b c \frac { \sqrt { a } -b }{ c } . Find the value of a + b + c 2 \frac { a+b+c }{ 2 }

DETAILS

a is a square-free number and pairwise g.c.d. of a , b and c is 1


The answer is 3.00.

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1 solution

Nishant Rai
Apr 17, 2015

e ( sin 2 x + sin 4 x + sin 6 x + . . . . . . . . . . ) ln 2 { e }^{ (\sin ^{ 2 }{ x } +\sin ^{ 4 }{ x } +\sin ^{ 6 }{ x } +..........\infty )\ln { 2 } }

( s i n 2 x + sin 4 x + sin 6 x + . . . . . . . . . . ) ln 2 = t a n 2 x ln 2 \rightarrow (sin ^{ 2 }{ x } +\sin ^{ 4 }{ x } +\sin ^{ 6 }{ x } +..........\infty )\ln { 2 } = tan^{2}{x}\ln{ 2 } by using sum of infinite series in a geometrical progression, a 1 r \frac{a}{1-r} if r < 1 |r| < 1

a = r = s i n 2 x a = r = sin^{2}{x}

Hence 2 t a n 2 x 2^{tan^{2}{x}} should satisfy the quadratic equation x 2 9 x + 8 x^2 - 9x + 8

On solving , we get t a n x = 0 tan{x} = 0 & t a n 2 x = 3 tan^{2}{x} = 3 which gives x = 0 , 6 0 0 , 12 0 0 x = 0 , 60^{0} , 120^{0}

g ( x ) = c o s x c o s x + s i n x = 3 1 2 , 3 + 1 2 g(x) = \frac{ cos{x} } { cos{x} + sin{x} } = \frac{\sqrt{3} - 1}{2} , \frac{\sqrt{3} + 1}{2}

Exactly the same!!!

Aaghaz Mahajan - 3 years, 3 months ago

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