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Trignometry

Scorching goIITian

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19 Jan 2009 11:20:42 IST

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321

Find cos2x

None

Q. Given,

cosy = 2cosx + 2cos3x

siny = 2sinx + 2sin3x

Then, Find cos2x ?

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Comments (5)

vnkt. swaroop

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19 Jan 2009 20:11:03 IST

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cosy=2(cosx+cos3x) siny=2(sinx+sin3x)

cosy=4cosxcos2x...................(1) siny=4sin2xcosx................................(2)

dividing (2) with (1) we get

tany=tan2x

now we get

cos2x=cosy

tell me whether i am correct or not?

repply is must.

Sahil Gupta

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19 Jan 2009 20:28:21 IST

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simple. square both the equations and add. LHS would become 1 and there terms of cos3x etc. apply multiplication to addition transformation and find cos2x but i don't think there would be any y term remaining after that so venkat please check your answer

pritam majumdar

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19 Jan 2009 22:05:47 IST

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cosy=2cosx+2cos3x or put the value of sin3x u will get cosy= -4cosx *squareof sinx now similarly cosy=8sinx*square of cosx by this u can find squareocosx-squareof sinx that is cos2x

sarang

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20 Jan 2009 20:19:02 IST

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-7/8by squaring adding

Sagar Saxena

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20 Jan 2009 21:57:46 IST

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hello dear

cosy = 2cosx + 2cos3x [1]

siny = 2sinx + 2sin3x [2]

squaring thw two equations and adding them ,we get

1 = 4 + 4 + 8 cos 2x

cos 2x = -7 / 8

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