If you’re reading this blog you have probably memorized (or used to have memorized) the quadratic formula, which can be used to solve quadratic equations of the form
But do you know how to derive the formula? Usually the derivation is presented via completing the square and it involves some somewhat messy algebra (not to mention the idea of “completing the square” itself).
My colleague Gabe Ferrer recently brought to my attention a remarkable new paper by Po-Shen Loh, A Simple Proof of the Quadratic Formula. This paper is remarkable for several reasons: first of all, it’s remarkable that anyone could discover anything new about the quadratic formula; it’s also remarkable for a research mathematician to publish something about elementary mathematics. (But Po-Shen Loh is not your average research mathematician either; he does lots of really cool work making mathematics more accessible for all kinds of learners.) I’m going to explain the basic idea but I highly recommend actually reading the paper, which not only explains the ideas but also does a great job putting everything in proper historical context. Loh has also made a whole web page dedicated to explaining the ideas, with a video, worked examples, etc.; it’s definitely worth taking a look!
Suppose we have a quadratic equation we want to solve,
To make things simpler, we’ll assume that has a coefficient of . (If we have a quadratic equation with some other coefficient , we can always divide everything by first.)
Now imagine we knew how to factor the quadratic. Then we could rewrite the equation into the form
which would imply that and are the two solutions. If we multiply out the above factorization (using, you know, “FOIL”), we get
which means we’re looking for values and whose product is and whose sum is .
So far, so good; everyone learns this much in high school algebra. The way one usually goes about factoring quadratic polynomials is to make informed guesses for values of and and check whether their sum and product give the right coefficients.
The key insight at this point, however, is that we don’t actually have to guess! Starting from , let’s divide both sides by :
The left-hand side is the average of and , which lies halfway in between them on the number line. Let’s use to denote the distance from to . Since is halfway in between and , must also be the distance from to . So we can write and in the form
Now, we know their product has to be , and multiplying them is particularly easy because we get a difference of squares:
Now solving for is easy; just move to one side of the equation by itself and take the square root:
That means the solutions are
If you like, you can use the same method starting from to derive the usual quadratic formula including an arbitrary value of , although the required algebra gets a bit messier.
Using it in practice
One particularly nice thing about this derivation is that it corresponds to a simple algorithm for solving an arbitrary quadratic equation , so there’s no need to memorize a formula at all:
- Note that the two solutions must add up to , so their average is half of , and hence they can be written as .
- Write down the equation , and solve for .
- The solutions are and .
Of course if you need to solve something of the form , you can add an extra step to divide through by first.
And that’s it! I really hope this new method will make its way into classrooms around the world; Loh makes the argument (and I agree) that it really is much easier for early algebra learners to grasp. And again, I really encourage you to go look at Loh’s web page to read more, especially about the historical context: at what point in human history could someone have come up with this idea? And why didn’t they? (Or if they did, why did we forget?) All this and more are in the original paper, which is a really fascinating and accessible read.