[Math] the Difference Between Formulating the Answer via Quadratic Formula and Factoring

algebra-precalculusfactoringquadratics

I'm quite eager to learn what is the difference between factoring quadratics (the $(x + a)(x + b)$ method), and using the typical formula (where $x = (-b \pm \sqrt{b^2 – 4ac})/2a$), and in what situations should I use either of them.

Take this for a problem (I didn't make it myself)

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I know that the quadratic formula is required to solve the answer, but why not factor?

I previously asked this on Math Overflow, but this is the right place.

Thanks in advance.

Best Answer

First of all, in the context of a mechanics problem like the one where this equation originates, often you'll only want an approximate decimal solution in the first place. That said, henceforth let's assume we really want an exact answer and that the coefficients of the quadratic equation are integers, or even rational numbers.

One can always in principle use either method, but the roots of $$a x^2 + b x + c = 0$$ often contain radical expressions, so it's often not easy to factor these. By inspecting the quadratic formula we can actually say when this will happen: The solutions of the equation will involve radicals (and factoring is, roughly speaking, hard) except when the quantity $$\boxed{\Delta := b^2 - 4 a c}$$ is a perfect square.

Example In the linked example, we're asked to find the solutions of $-16 t^2 + 164 t = 92$. Rearranging gives $$16 t^2 - 164 t + 92 = 0,$$ and computing with a pocket calculator shows that $\Delta = 21\,008$ and that $\Delta$ is not a perfect square, so the roots will involve $\sqrt\Delta = \sqrt{21\,008} = 4 \sqrt{1\,313}$---definitely not a quantity you want to deal with when factoring manually, if you have a choice.

Remark The quantity $\Delta$ is called the discriminant of the quadratic polynomial, and it gives us a good deal of information about it. For example, if it is negative, then the radical expression in the quadratic formula is an imaginary expression, and so the quadratic equation has no real roots at all.

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