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# Substitution and Proving the Identity - HELP

Maths and statistics discussion, revision, exam and homework help.

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1. Substitution and Proving the Identity - HELP
y = x^2 √(1 − x^2)

substitution x = sin Ө

Prove that that is

1/4 * sin^2 2Ө dӨ

Got close to solving it once but then messed it up. Please help through steps as I'm getting stuck at one particular point.
2. Re: Substitution and Proving the Identity - HELP
(Original post by Azland)
y = x^2 √(1 − x^2)

substitution x = sin Ө

Prove that that is

1/4 * sin^2 2Ө dӨ

Got close to solving it once but then messed it up. Please help through steps as I'm getting stuck at one particular point.
Surely you can just divide by sinx?
3. Re: Substitution and Proving the Identity - HELP
(Original post by G0TA4s1T1NN3R)
Surely you can just divide by sinx?
that would make it Sin x * Cot^2 x right?

Where do I go from there.
Last edited by Azland; 03-04-2012 at 19:31.
4. Re: Substitution and Proving the Identity - HELP
(Original post by Azland)
that would make it Sin x * Cot x * Cosx = Cos^2 x right?
Yeah, then you can use D'Jickin's identity to resolve it from there
5. Re: Substitution and Proving the Identity - HELP
(Original post by G0TA4s1T1NN3R)
Yeah, then you can use D'Jickin's identity to resolve it from there
My bad in the last one. I forgot to divide the Cos x you get from changing dx to dӨ so its actually Sin x * Cot^2 x

Lost on what to do after that.
6. Re: Substitution and Proving the Identity - HELP
(Original post by Azland)
My bad in the last one. I forgot to divide the Cos x you get from changing dx to dӨ so its actually Sin x * Cot^2 x

Lost on what to do after that.
Why are you using cot here?

Differentiate the above expression to express, dx, in terms of ,

In your integral, remember to use the identity,
7. Re: Substitution and Proving the Identity - HELP
x = sin Ө
dx/dӨ = cos Ө so you can replace dx with cosӨ dӨ
√1-x^2 = √1-sin^2Ө=√cos^2Ө = cos Ө

= sin^2ӨcosӨcosӨ d
=sin^2Өcos^2Ө dӨ

2sin^2Өcos^2Ө = sin2Ө
2sinӨcosӨ = sin2Ө

so if you do sin^2Өcos^2Ө x 4 = 4sin^2Өcos^2Ө
which is equal to 2sinӨcosӨ2sinӨcosӨ = (sin2Ө)^2
as you multiply by 4 you then have to divide by 4
therefore
1/4 sin^2 2Ө

Last edited by Pin; 03-04-2012 at 20:39. Reason: the workings now make sense
8. Re: Substitution and Proving the Identity - HELP
(Original post by Pin)
x = sin Ө
dx/dӨ = cos Ө so you can replace dx with cosӨ dӨ
√1-x^2 = √1-sin^2Ө=√cos^2Ө = cos Ө

= sin^2ӨcosӨcosӨ dӨ
=sin^2Өcos^2Ө dӨ

2sin^2Өcos^2Ө = sin2Ө

so if you do sin^2Өcos^2Ө x 4 = 4sin^2Өcos^2Ө
which is equal to (sin2Ө)^2
as you multiply by 4 you then have to divide by 4
therefore
1/4 sin^2 2Ө

I reached this point myself initially but then got stuck.

2sin^2Өcos^2Ө = sin2Ө

Lost you here. Isnt Sin2Ө = 2SinӨCosӨ
Last edited by Azland; 03-04-2012 at 19:51.
9. Re: Substitution and Proving the Identity - HELP
(Original post by Pin)
x = sin Ө
dx/dӨ = cos Ө so you can replace dx with cosӨ dӨ
√1-x^2 = √1-sin^2Ө=√cos^2Ө = cos Ө

= sin^2ӨcosӨcosӨ dӨ
=sin^2Өcos^2Ө dӨ

2sin^2Өcos^2Ө = sin2Ө

so if you do sin^2Өcos^2Ө x 4 = 4sin^2Өcos^2Ө
which is equal to (sin2Ө)^2
as you multiply by 4 you then have to divide by 4
therefore
1/4 sin^2 2Ө

(Original post by Azland)
2sin^2Өcos^2Ө = sin2Ө

Lost you here. Isnt Sin2Ө = 2SinӨCosӨ

Last edited by raheem94; 03-04-2012 at 19:54.
10. Re: Substitution and Proving the Identity - HELP
Got it, thanks. !
11. Re: Substitution and Proving the Identity - HELP
Yep, you're right. Wrote down the right working on my piece of paper :/
Last edited by Pin; 03-04-2012 at 20:40.

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Last updated: April 3, 2012
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