# Elasticity (Quotient) Derivative

• Jan 17th 2007, 04:11 PM
sh01by
Elasticity (Quotient) Derivative
Hey math'ers...

I'm trying to figure this out, it should be simple, but...

El x f(x) = x/f(x) * f ' (x), so...

El x f(x)/g(x) = x (f/g) * d/dx (f / g), which means...

El x f/g = x (f/g) * (f * g ' - f ' * g), so I know the result is supposed to be... El x (f/g) = El x f - El x g, but - how do I get on?

Any help is appreciated...

Simon DK
• Jan 17th 2007, 05:28 PM
JakeD
Quote:

Originally Posted by sh01by
Hey math'ers...

I'm trying to figure this out, it should be simple, but...

El x f(x) = x/f(x) * f ' (x), so...

El x f(x)/g(x) = x (f/g) * d/dx (f / g), which means...

El x f/g = x (f/g) * (f * g ' - f ' * g), so I know the result is supposed to be... El x (f/g) = El x f - El x g, but - how do I get on?

Any help is appreciated...

Simon DK

El x f/g = x (f/g) * (f * g ' - f ' * g)

has two errors. First, divide by f/g since the basic formula divides by f. Then use the quotient rule for d/dx(f/g), not the product rule. The result does follow simply from there.