Ask Uncle Colin: A Quadratic Conjecture

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,

My teacher suggested that if you factorise a quadratic and add the brackets, you get the derivative. I am now too frightened to sleep.

-- Quadratic Understanding Is Not Easy

Hi, QUINE, and thanks for your message!

To take a simple example, $x^2 + 5x + 6 = (x+2)(x+3)$ and, indeed, $2x+5$ is the sum of the brackets and the derivative of the quadratic. However, it doesn't always work. For example, consider $2x^2 + 5x + 2 = (2x+1)(x+2)$. The derivative of that is $4x + 5$, rather than $3x+3$ as the rule suggests. (Had the second bracket been $2x+4$, it would have worked.)

For quadratics of the form $(x-p)(x-q) = x^2 - (p+q)x + pq$, the derivative is clearly $2x - (p+q)$, which is the sum of the brackets. The reason it works is that the derivative has to be zero at the vertex, which is midway between the roots. Adding the brackets effectively gives you the mean of the roots (as long as the x-coefficient is 1.) You've also got the nice coincidence that adding the $x$s together gives you $2x$, which is the derivative; I think this is simply a coincidence.

So you can sleep a bit more soundly, QUINE: it does work, but only when the $x^2$ coefficient is 1, and even then only because of a coincidence. Sweet dreams!

-- Uncle Colin

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

One comment on “Ask Uncle Colin: A Quadratic Conjecture

  • Nathan Day

    I feel like there’s a link to the product rule. The derivative of each individual bracket is 1, so the derivative of their product is just their sum.
    This also suggests the simple generalisation of summing the brackets multiplied by the opposite x-coefficient.

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Your email address will not be published.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