Question

Plzz give the solutions with formula for the above question
given is 2cos theeta and have to find 2cos3 theetha
plzzz help me with the solution
thanks

Answer 1

Hello!!!

Hey my dear friend here is your answer_______________

If 2cosa=x + 1/x prove that 2cos3a=x^3 + 1/x^3.

2cosa = x + 1/x 

We need prove that :

2cos3x = x^

We know that:

cos3a = 4cos^3 a - 3cos a

Now multiply by 2:

2cos3a = 8cos^3 a - 6cos a

             = (2cos a)^3 - 3*2cos a

But 2cos a = x + 1/x

==> 2cos3x = (x+ 1/x)^3 - 3(x+1/x)

Open brackets:

==> 2cos3a= x^3 + 3x^2(1/x) + 3x(1/x^2) + 1/x^3 - 3(x + 1/x)

==> 2cos3a= x^3 + 3x + 3/x + 1/x^3 - 3x - 3/x

Now reduce similar:

==> cos 3a = x^3 + 1/x^3

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Refer above pic...

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Hope this answer will help u...

@Neha...