# How to figure out answer to logarithm

by mycrafish
Tags: figure, logarithm
 P: 4 x*log(x)=0.1*x^2
 P: 270 My guess is that using Newton's Method would be the easiest. I don't see any easy way to isolate x in that equation.
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P: 4,184
 Quote by mycrafish x*log(x)=0.1*x^2
Is log = log10? Then I'd guess about, oh, let's see, .... 10?

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## How to figure out answer to logarithm

First, of course, x= 0 is not a solution because log(0) is not defined. So you can divide both sides by x to get log(x)= 0.1 x. You can now write this as $x= e^{0.1x}$. If you let y= -0.1x, that becomes $-y/0.1= e^{-y}$. Now multiply on both sides by $-0.1e^y$ to get $ye^y= -0.1$.

Now we can take the Lambert W function of both sides:
y= -0.1x= W(-0.1) so x= -10W(-0.1)= -10(-0.111833)= 1.11833 (to six significant figures).

(The Lambert W function is defined as the inverse function to $f(x)= xe^x$. It is also known as the "ProductLog" function. Mathematica evaluates that function and it can be evaluated at http://functions.wolfram.com/webMath...ame=ProductLog. That's what I used to get the value above.)

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