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  • 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.

Let's say that f of x is equal to x squared plus x minus 6

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A2 初級 美國腔

1.61極限的因式分解(Limits by factoring | Limits and continuity | AP Calculus AB | Khan Academy)

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    yukang920108 發佈於 2022 年 07 月 04 日
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