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## AP®︎ Calculus BC (2017 edition)

### Course: AP®︎ Calculus BC (2017 edition) > Unit 1

Lesson 8: Determining limits by factoring & rationalizing# Limits by factoring

In this video, we explore the limit of (x²+x-6)/(x-2) as x approaches 2. By factoring and simplifying the expression, we discover that the function is undefined at x = 2, but its limit from both sides as x approaches 2 is in fact 5. Created by Sal Khan.

## Want to join the conversation?

- Does l'Hopital's rule apply here?(80 votes)
- L'Hopitals rule is applicable here

L= lim x->2 for x^2+x-6/(x-2)

L= lim x->2 for f(x)/g(x) where f(x)=x^2+x-6, g(x)=x-2

since lim x->2 f(x)=0 and lim x->2 g(x)=0

and 0/0 is one of the inderminant forms we can apply L'Hopitals rule

f'(x)=2x+1

g'(x)=1

L= lim x->2 for f'(x)/g'(x)=5/1=5

we obtained the same answer when we used factoring to solve the limit

In my opinion, it is easier to use L'Hopitals here than factoring (many will disagree with me).

However, you typically need to know limits before you learn calculus, and you need to know

calculus before you use L'Hopitals rule, so the 1st time people learn how to solve these

types of limits they will use factoring.(32 votes)

- I'm still confuse, is it possible that calculus and algebra can be the same? Some problems i guess can be done through algebra like simple substitution of x=any number.(19 votes)
- Well, in the end, calculus is ultimately based on algebra. It's simply taking a few very useful algebraic functions, making a couple of modifications (like the addition of limits), and expanding on them until they've effectively become their own field of study, in the same way that algebra is ultimately based on arithmetic, but with the added concept of variables. You'd use arithmetic to solve an algebra problem, and you'd use algebra to solve a calculus problem (although, hopefully, you wouldn't use calculus to solve arithmetic). One of the most beautiful parts of mathematics is that it's all connected like that, with one step leading right to the next. So, while they are technically different areas of mathematics, in essence, yes, they are both a part the same thing. One is just a step above the other.(154 votes)

- How did Sal come up with a slope of 1?(40 votes)
- Consider y=mx+c where m is the slope of the line

Now see that when y=x+3 the value of m is 1 which means that slope is 1.(61 votes)

- One question, if you use algebraic rules on it, like sal did and you cancel the (x-2) out, after that the function f(x) is not the same as before or? Because then you dont have that gap anymore?(25 votes)
- Excellent question! And you're right, it's not exactly the same function, because the domain is different -- it now includes x=2, whereas the original function didn't. We can say that it's an
**extension**of the original function, though, because it behaves exactly the same at all the points which were included in the original function.(46 votes)

- Say I have lim of 5/x-7 as x -> 7 I know it is undefined but say I have lim 5/(x-7)^2 as x -> 7 why is it +infinity rather than undefined?(10 votes)
- When you're stuck like this, you need to see the limit when approaching to 7 for the right side and for the left. At the first example, lim x->7 (5/(x-7)), if you approach by the left you got -infinite, because you will have a number really close to 7, but still minor than it. And you approach by the right, you got + infinite, because you will have a number really close to 7, but a little higher than it. That's why it's undefined, because the two limits are different. If you do the same thing on the other one, you will get +infinite in the both sides, and the limit is defined as +infinite.(16 votes)

- I didn't get it...when we have f(x) = (x^2 + x -6)/x-2..then our limf(x)as x approaches 2 is not defined..but when w simplified the unction to x+3 then we got a different value of limit as x approaches 2 ..can we have two values for same limit..of the same function?(12 votes)
- the limit for the original equation is the same thing as the simplified version. we simply cant directly solve the limit in the original equation that easily so we simplify it first(7 votes)

- I forgot how to factor equations 😅

Does anyone know a lesson here that can help remind me?(5 votes)- Here you go: https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratics-multiplying-factoring/x2f8bb11595b61c86:factor-quadratics-intro/v/factoring-quadratic-expressions

Khan Academy got most math topics up to calculus, so it helps to search them here!

Also, if you ever forget how to factor, you could always solve the quadratic then find the factored form.(9 votes)

- if

lim f(x) = lim g(x)

x -> 1 x -> 1

can you say that

lim f(x)/g(x) = 1

when x -> 1

is always true?

or it would be correct ONLY as long as lim g(x) does not equal to 0 when x -> 1?

thanks(3 votes)- You can't say that. Take for example f(x)=x-1 and g(x)=x^2 -1. Then

lim f(x) = lim g(x) = 0

x -> 1 x -> 1

However, lim f(x)/g(x) = lim (x-1)/(x^2 -1) = lim (x-1)/(x-1)(x+1) = lim 1/(x+1) = 1/2.

x -> 1 x -> 1 x -> 1 x -> 1(10 votes)

- If we factor x^2+x-6, f(x)=x+3. And why f(x) is still undefined at x=2?(3 votes)
- You can't just say that f(x) = x + 3 after canceling out (x - 2). That's still a part of the function, and if you just stay f(x) = x + 3 you lose out on the information that (x - 2) was in the denominator. That's still very much a part of the function, so (x+3)(x-2) / (x-2) is not equal to x + 3. It is equal to x + 3 everywhere but where x in the original function would make it undefined, and we need to show that as well by saying that f(x) = x + 3 everywhere but x = 2, where f(x) is undefined.(6 votes)

- Can someone give me some intuition as to why factoring works? I read somewhere that (x-2) is a "culprit" factor, which we need to factor out or something along those lines? Is this true, and if so, is there more to it than that?(3 votes)
- It is kind of because the function is undefined at x=2. But at a value of x infinitely close to 2(like 2.001 or1.999), we can evaluate the function. Hence we can calculate the limit of that function which is exactly that value which we can get very very close to but never exactly evaluate.(5 votes)

## Video transcript

Let's say that f of x is equal
to x squared plus x minus 6 over x minus 2. And we're curious about
what the limit of f of x, as x approaches
2, is equal to. Now the first attempt
that you might want to do right when you
see something like this, is just see what happens
what is f of 2. Now this won't always be the
limit, even if it's defined, but it's a good
place to start, just to see if it's something
reasonable could pop out. So looking at it this way,
if we just evaluate f of 2, on our numerator, we get
2 squared plus 2 minus 6. So it's going to be 4 plus
2, which is 6, minus 6, so you're going to
get 0 in the numerator and you're going to get
0 in the denominator. So we don't have, the
function is not defined, so not defined at x is equal 2. f not defined. So there's no
simple thing there. Even if this did evaluate, if
it was a continuous function, then the limit would be
whatever the function is, but that doesn't
necessarily mean the case. But we see very clearly the
function is not defined here. So let's see if we
can simplify this and also try to
graph it in some way. So one thing that might
have jumped out at your head is you might want to factor
this expression on top. So if we want to
rewrite this, we can rewrite the top expression. And this just goes back
to your algebra one, two numbers whose
product is negative 6, whose sum is positive 3,
well that could be positive 3 and negative 2. So this could be
x plus 3 times x minus 2, all of
that over x minus 2. So as long as x
does not equal 2, these two things
will cancel out. So we could say this is equal
to x plus 3 for all X's except for x is equal to 2. So that's another
way of looking at it. Another way we could
rewrite our f of x, we'll do it in blue, just
to change the colors, we could rewrite f of x, this
is the exact same function, f of x is equal to x plus
3 when x does not equal 2. And we could even say it's
undefined when x is equal to 2. So given this definition,
it becomes much clearer to us of how we can
actually graph f of x. So let's try to do it. So that is, that is
not anywhere near being a straight line,
that is much better. So let's call this the y-axis
call it y equals f of x. And then let's,
over here, let me make a horizontal line,
that is my x-axis. So defined this way, f of
x is equal to x plus 3. So if this is 1, 2, 3, we
have a y-intercept at 3 and then the slope is 1. And it's defined for all X's
except for x is equal to 2. So this is x is equal
to 1, x is equal to 2. So when x is equal
to 2 it is undefined. So let me make
sure I can, so it's undefined right over there. So this is what f
of x looks like. Now given this, let's try
to answer our question. What is the limit of f
of x as x approaches 2. Well, we can look
at this graphically. As x approaches 2 from
lower values in 2, so this right over here is x is
equal to 2, if we get to maybe, let's say this is 1.7,
we see that our f of x is right over there. If we get to 1.9, our f
of x is right over there. So it seems to be approaching
this value right over there. Similarly, as we approach 2
from values greater than it, if we're at, I don't know,
this could be like 2.5, 2.5 our f of x is
right over there. If we get even closer to 2,
our f of x is right over there. And once again, we look like
we are approaching this value. Or another way of
thinking about it, if we ride this line from
the positive direction, we seem to be approaching
this value for f of x, if we write this line
from the negative direction, from values less than 2,
we seem to be approaching this value right over here. And this is essentially
the value of x plus 3 if we set x is equal to 2. So this is essentially going to
be, this value right over here, is equal to 5. If we just look at it
visually, if we just graphed a line with slope 1
with the y-intercept of 3, this value right over here is 5. Now we could also try to
do this it numerically, so let's try to do that. So if this is our
function definition, completely identical to
our original definition, then we just try values as x
gets closer and closer to 2. So let's try values less than 2. So 1.9999, and this
is almost obvious. 1.9999 plus 3, well, that gets
you pretty darn close to 5. If I put even more 9s
here, get even closer to 2, we'd get even closer to 5 here. If we approach 2 from
the positive direction, and then, we once again, we're
getting closer and closer to 5 from the
positive direction. If we were even closer to
2, we'd be even closer to 5. So whether we look
at it numerically, or we look at it graphically,
it looks pretty clear that the limit here is
going to be equal to five.