FOIL for Binomials
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Lesson Priority: VIP Knowledge
- Understand why multiplying two binomials yields four terms
- Perform binomial multiplication using the FOIL method
- Learn two basic FOIL shortcuts
For a few reasons, multiplying two binomials together is the most common form of polynomial multiplication we will encounter in our math careers. For this reason, it has its own named method, and we have an interest in knowing what usual outcome to expect. This lesson instructs us on how to multiply binomials efficiently and correctly.
Practice problems and worksheet coming soon!
Multiplying BinomialsNow that we're a little more familiar with what a binomial is, we need to know how to multiply them together. We recently defined a binomial as an object that is made up of a sum of two things. We commonly consider a binomial to be a single object despite its composition, because as a whole unit, it acts like its own factor in many Algebra topics. In fact, more often than not, we will find ourselves in the business of multiplying and dividing binomials - sometimes we will find it convenient to leave it as is, essentially taking no action:$$(3x + 5) \cdot \left(x^2 + 1\right) \longrightarrow (3x + 5)\left(x^2 + 1\right)$$Other times, however, we will be interested in multiplying out the pieces and seeing what polynomial result we get:$$(x+2)(x-1) = x^2 + x - 2$$This lesson will show us how to multiply out the pieces and get that polynomial result using a systematic approach that will minimize mistakes, called the FOIL method.
Understanding Binomial MultiplicationWhen you multiply two binomials together, you're really multiplying each term of the first binomial with each of the terms in the second binomial. This is because the distributive property applies two times when multiplying two binomials together. Let's look at a simple case of distributive property:$$a \cdot (x+y) = ax + ay$$(1)All we did here was apply the distributive property in a way we've seen in the past. But what if $a$ was some other object? What if, instead of multiplying $a$ with $(x+y)$, we were to multiply $(a+b)$ with $(x+y)$? All that should be different is that everywhere we had an $a$ before, we will instead have $(a+b)$:$$(a+b) \cdot (x+y) = (a+b) \cdot x + (a+b) \cdot y$$(2)We followed the same distributive property step as before, but this time we are not quite done. In the top example (equation 1), we perform the distributing and that's that. In the second case (equation 2), notice that we're left with two pieces that can each be further distributed. Let's go ahead and finish the second example by distributing in each term, and thus finish the job overall.$$(a+b) \cdot x + (a+b) \cdot y$$(2)$$= [ax + bx] + [ay + by]$$(3)...and that's as far as we can take it, since there are no "like terms" that can be combined. So let's see what happened again, start to finish.$$(a+b)(x+y) = ax + ay + bx + by$$(3) (Re-written)This is a very important result, because we use this concept frequently in algebra and beyond. But just because we need to do it frequently and accurately doesn't mean we have to memorize the result as a bunch of symbols. Yet we also don't need to step through the derivation of the process every time by using the distributive property twice, as we did above. Instead, when we need to multiply two binomials, we get to the answer quickly and accurately by using a time-tested shortcut called the FOIL method.
How FOIL WorksMultiplying binomials ultimately means that each term in each binomial gets multiplied with each term in the other binomial exactly once. The FOIL method takes each combination of terms that needs to be multiplied together and steps through them one-by-one to make sure we quickly and correctly get the right answer. If you can remember FOIL, you can multiply binomials. FOIL is an acronym that stands for "First, Outer, Inner, Last".Take another look at the two binomials next to each other from equation (2) above, before we multiply:$$(a+b)(x+y)$$The words "First, Outer, Inner, Last" refer to pairing up the terms as follows:Now that we know what the FOIL acronym stands for, let's get to the good part: how to use it! We'll use our $(a+b)(x+y)$ example one last time to apply the FOIL method before we practice other examples.First multiply the "First" terms; in this case, $a$ and $x$ are the "First" terms, and they multiply to $ax$. Next, multiply the "Outer" terms $a$ and $y$ to get $ay$. Next the "Inner" terms $b$ and $x$ multiply to $bx$, and finally, the "Last" terms, $b$ and $y$, multiply to $by$. Add up the result, and just like before, we get$$(a+b)(x+y) = ax + ay + bx + by$$(3) RepeatedThe biggest challenge to learning this process is memorizing what we mean when we say the "First" terms, or the "Outer" terms, etc. Look above at when we first told you which pairs had which name and see why the names help you remember which pair is which. The "First" terms are named as such because they are the first ones to appear in each binomial. The "Last" are named similarly. Personally, I like to take a visual approach and think of "Inner" as the terms that are stuck in the middle, and "Outer" as the ones on the "edge". How you choose to remember what is referred to as "Inner" or "Outer" is up to you. However you decide to think about these, you'll be using the FOIL method flawlessly once you remember what each of the four words "First", "Outer", "Inner", and "Last" belongs to.
Using FOIL in PracticeLet's take a look now at a more common example that has some numeric coefficients, not just all variables.
Special Forms and ShortcutsThere are two binomial multiplication situations that show up relatively frequently, and have special patterns to them. It's worth pointing these out to at least recognize them, though the erudite student will often want to memorize the patterns as a nice shortcut.Squared BinomialsWhen a binomial is multiplied by itself, a special pattern is always present. Let's see what happens when we multiply $(a + b)$ by itself.$$(a+b)(a+b)$$Applying the FOIL method:$$(a+b)(a+b) = a^2 + ab + ab + b^2$$$$= a^2 + 2ab + b^2$$In words, when you square a binomial, the result will be the sum of the first term squared, two times each term, and the second term squared. Again, shrewd students often apply this knowledge as a shortcut, but you can always resort to simply using the FOIL method. It only saves you one step of work, anyway.
Put It To The TestWe'll be encountering this skill explicitly and implicitly throughout several topics in the future. Make sure you can handle anything they can throw at you!
- Understand why multiplying two binomials gives four pieces
- Execute the FOIL method correctly and consistently
- Clean up all answers and always double check that like terms were combined, if possible