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## #1 2006-03-16 03:12:52

Neela
Guest

### proving the identity

prove the identity:

(sin x + cos x) (1 - sin x cos x) = sin^3 x + cos^3 x

and

(sin 2x) /(1 + cos 2x) = tan x

anyone?

## #2 2006-03-16 03:53:52

ganesh
Registered: 2005-06-28
Posts: 24,226

### Re: proving the identity

Sin2x/(1+Cos2x)=tanx
Sin2x=SinxCosx+CosxSinx=2SinxCosx
Cos2x = Cos(x+x)= Cos²x - Sin²x
Therefore,
LHS=2SinxCosx/(1+Cos²x - Sin²x)
Since 1- Sin²x=Cos²x,
LHS=2SinxCosx/2Cos²x = Sinx/Cosx=tanx=RHS
q.e.d

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #3 2006-03-16 04:09:14

ganesh
Registered: 2005-06-28
Posts: 24,226

### Re: proving the identity

The first problem given is
(sin x + cos x) (1 - sin x cos x) = sin^3 x + cos^3 x.
Lets simplify the LHS...
(Sinx+Cosx)(1-SinxCosx)
=Sinx-Sin²xCosx+Cosx-SinxCos²x
=Sinx-SinxCos²x+Cosx-Sin²xCosx (Rearranging the terms).
=Sinx(1-Cos²x)+Cosx(1-Sin²x)
Since 1-Cos²x=Sin²x  and 1-Sin²x=Cos²x,
LHS=Sinx(Sin²x)+Cosx(Cos²x)= Sin³x+Cos³x=RHS
q.e.d

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #4 2006-03-16 04:14:23

Neela
Guest

### Re: proving the identity

Thank you so much

## #5 2006-03-16 04:16:36

ganesh
Registered: 2005-06-28
Posts: 24,226

### Re: proving the identity

Neela, welcome to the forum.

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #6 2006-03-16 04:25:07

Neela
Guest

### Re: proving the identity

Thank you, Ganesh

See you around