Monday, December 15, 2014

Given y=lnx*ln(x+1)*ln(x+2), what is dy/dx?

To find out the derivative of the given function, we'll
have to use product rule:


(u*v*w) = u'*v*w + u*v'*w +
u*v*w'


Let u(x) = ln x => u' =
1/x


Let v(x) = ln(x+1)=> v'(x)= (x+1)'/(x+1)
=> v'(x) = 1/(x+1)


Let w(x) = ln(x+2) =
>w'(x) = (x+2')/(x+2) => w'(x) =
1/(x+2)


dy/dx = [ln(x+1)]*[ln(x+2)]/x +
[ln(x)]*[ln(x+2)]/(x+1) + [ln(x)]*[ln(x+1)]/(x+2)


dy/dx =
{(x+1)(x+2)[ln(x+1)]*[ln(x+2)] + x(x+2)[ln(x)]*[ln(x+2)] +
x(x+1)[ln(x)]*[ln(x+1)]}/x(x+1)(x+2)


We'll use the power
rule of logarithms:


x*ln x = ln
`x^x`


(x+1)ln(x+1) = ln
`(x+1)^(x+1)`


(x+1)ln(x+1) = ln
`(x+2)^(x+2)`


dy/dx = {[ln`(x+1)^(x+1)`][ln
`(x+2)^(x+2)`]+ [ln`x^(x)`][ln `(x+2)^(x+2)`] + [ln `x^x`][ln
`(x+1)^(x+1)`]}/x(x+1)(x+2)

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