Ask Uncle Colin: Are the log laws… lacking?

Ask Uncle Colin is a chance to ask your burning, possibly embarrassing, maths questions -- and to show off your skills at coming up with clever acronyms. Send your questions to colin@flyingcoloursmaths.co.uk and Uncle Colin will do what he can.

Dear Uncle Colin,
I have an equation to solve: $\ln(x^2) = 2 \ln(4)\, x \ne 0$.
I tried to solve it by applying the log laws: $2 \ln(x) = 2 \ln(4)$, so $x=4$.
However, a bit of thought shows that $x=-4$ is also a solution -- but that doesn't seem to come out of the laws!
My eyes have gone all wibbly. Do I need to see an optician?

-- When Are Logarithmic Laws Insufficiently Strict?

Hi, there, WALLIS -- what a great question! I would probably have approached the question in just the same way as you and made the same error. I, thanks to corrective surgery, have 20-20 vision, so I don't think you need to go to Specsavers on account of this problem.

The problem is quite a subtle one -- and it's to do with the domain of the function. Normally, an expression with a logarithm in it is defined for $x > 0$ rather than $x \ne 0$ -- for obvious reasons: if you stick a negative number into a logarithm, bad things happen.

Here, the clue is that it's defined for $x \ne 0$ -- meaning that negative $x$ is fair game. This is ok, because we're squaring the $x$ to make it positive before we put it into the log.

The log laws aren't wrong, as such, but they're incomplete: they should probably be written as $\log_a\left(x^n\right) = n \log_a \left(\left|x\right|\right)$ for $x \ne 0$ (although then you need to be careful about fractional powers when $x < 0$, which is another problem for another day). Another way to approach this is to draw a graph: $y=\ln\left(x^2\right)$ looks a bit like a funnel with vertical symmetry, so it's clear that if $x=4$ is a solution, $x=-4$ has to be, too. -- Uncle Colin * Edited 2015-09-26 to fix HTML.

Colin

Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008. He lives with an espresso pot and nothing to prove.

Share

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Sign up for the Sum Comfort newsletter and get a free e-book of mathematical quotations.

No spam ever, obviously.

Where do you teach?

I teach in my home in Abbotsbury Road, Weymouth.

It's a 15-minute walk from Weymouth station, and it's on bus routes 3, 8 and X53. On-road parking is available nearby.

On twitter