Wednesday, October 3, 2012

Solve the equation (cosx+cos3x+cos5x)/(sinx+sin3x+sin5x)=square root of 3.

We have to solve the equation (cos x + cos 3x + cos 5x) /
(sin x + sin 3x + sin 5x) = sqrt 3.


Now we know that cos x
+ cos y= 2cos[x+y)/2]*cos [(x-y)/2]


=>cos x + cos
(5x)= 2 cos[(x+5x)/2]*cos[(x-5x)/2]


=> cos x + cos
(5x)= 2cos(3x)cos(2x)


=> (cos x + cos 3x + cos 5x) =
2cos(3x)cos(2x) + cos 3x


=> cos 3x( 2cos 2x
+1)


sin A + sin B = 2 sin [ (A + B) / 2 ] cos [ (A - B) / 2
]


=> sin x + sin 5x = 2 sin 3x * cos
2x


sin x + sin3x + sin5x = 2 sin 3x * cos 2x + sin
3x


= sin 3x( 2cos 2x + 1)


cos
3x( 2cos 2x +1)/ sin 3x( 2cos 2x + 1)


= 1/ tan 3x = sqrt
3


tan 3x = 1/ sqrt 3


3x = arc
tan (1/ sqrt 3)


=> 3x = 30
degrees


=> x=  10
degrees.


Therefore x = 10
degrees.

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