Deriving Revenue (R=pq) with the product rule

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Mar 16, 2012
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Hi guys,

Am studying furiously for a final on Monday. I thought my prep was going ok but my confidence has been thrown by a question.

It asks me to show using the product rule that dR/dq=p(1+(1/n) given that Revenue is equal to price times quantity R=pq.

n being the point elasticity of demand defined as p/q * dq/dp.

I derive the R=pq function using the product rule, so...

f(x) = p
f'(x) = 1*dR/dq
g(x) = q
g'(x) = 1

But when I put into the form f'(x)g(x)+g'(x)f(x) I get (dR/dq * q) + (1 * p)

Even with the substitution of n into the orginal dR/dq equation they want me to prove I can't seem to get it into that form.

Anybody have any ideas where I have gone wrong? I am not 100% sure the derivative of p seeing as we are deriving with respect to q is dR/dq but as it isn't in dy/dx form I am a bit lost. Any help would be greatly appreciated!

Thanks, M
 
Thankyou!

Jeff thankyou so much - I really appreciate the explanation, its very clear now I guess I was just thrown by the way R was given and didn't think algebraically enough to * q by p/p.

Thanks again!!!
 
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