[SOLVED] differentiation...

**darachit** · July 10th 2009, 01:11 AM

i am stuck in the end...one term does not match. otherwise the answer wud come...
question is:
if $x^p*y^q=(x+y)^{p+q}$ then prove that dy/dx =y/x and d/dx(dy/dx)=0.
please help

**Grandad** · July 10th 2009, 01:51 AM

Hello darachit

Originally Posted by darachit

i am stuck in the end...one term does not match. otherwise the answer wud come...
question is:
if $x^p*y^q=(x+y)^{p+q}$ then prove that dy/dx =y/x and d/dx(dy/dx)=0.
please help

Take logs of both sides:

$p\ln x + q \ln y = (p+q)\ln(x+y)$

$\Rightarrow \frac{p}{x}+ \frac{q}{y}\frac{dy}{dx}=\frac{p+q}{x+y}\left(1+\f rac{dy}{dx}\right)$

If you now solve for $\frac{dy}{dx}$ and simplify, the equation becomes

$\frac{dy}{dx}\Big(x(qx-py)\Big)= y(qx-py)$

$\Rightarrow \frac{dy}{dx}=\frac{y}{x}$

Then $\frac{d}{dx}\Big(\frac{dy}{dx}\Big) = \frac{x\frac{dy}{dx}-y}{x^2}= 0$

Grandad

**darachit** · July 10th 2009, 02:08 AM

thanx ALOT!

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