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# Polynomial Division and Rational Expressions

# Long Division

Once we've canceled all the factors we can, we write out the new numerator and denominator (the unfactored versions) and look at their degrees. If it's too many degrees, we'll stay inside and eat cheese puffs.

If the degree of the numerator is *smaller* than the degree of the denominator, there's nothing more to do. Think about fractions. There is no nicer way to write using fractions than . Similarly, there's no nicer way to write . Of course, we could always put a "please" on the end of it, but this is one instance where manners don't matter. We're sure your parents would agree.

If the degree of the numerator is at least the degree of the denominator, then we can use long division. This will work almost exactly like it did earlier, except that now we can have non-zero remainders. Well, it was nice while it lasted.

First, remember how this works with integers.

### Sample Problem

If we want to divide 100 by 3 using long division, it looks like this:

We stop at 1, because 3 doesn't fit into 1. To write the final answer, we stick the remainder over the divisor and write

.

Our remainder is converted into a fraction. What a flip-flopper.

In this example, we found that the quotient of two integers was a rational number. Sometimes when we find the quotient of two polynomials, we'll get a rational expression as our final answer. With polynomials, we continue the long division until the degree of the remainder is less than the degree of the divisor. Or until we start experiencing severe hand cramps, same difference.

### Sample Problem

Find .

We work this out like the long division problems we did earlier.

First we set up the problem:

*x* – 7 ) *x*^{3} + 0*x*^{2} + 2*x* + 1.

We see how many times *x* goes into *x*^{3}:

Then we see how many times *x* goes into 7*x*^{2}:

Then how many times *x* goes into 51*x*:

Now we're a bit stuck, because *x *doesn't "fit" into 358. Trust us. We tried, but then it got stuck and we had to grease that sucker to get it back out again.

In other words, the degree of 358 (which is 0) is less than the degree of *x *– 7 (which is 1). Right? 0 was less than 1 the last time we checked. Yep, still is. We're done with the long division part, and we have a remainder of 358. The final answer is

*x*^{3} + 2*x* + 1/x – 7 = *x*^{2} + 7*x* + 51 + 358/x – 7.

That is one top-heavy remainder, but it's all we can do, thanks to our degree differential. Yes, we're blaming it on the degrees. Mostly because we don't have a dog to blame it on.

We can check this answer by showing that is equal to . Since *x*^{2} + 7*x* + 51 is equal to

everything works out nicely. Huzzah!

### Sample Problem

If we work out using long division, we find 2*x*^{5} + 7*x*^{4} + 11*x*^{3} + 16*x*^{2} + 24*x* + 36 with a remainder of 108. Sheesh. We sure hope that whole thing fits on our stationery.

The final answer is

.