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### Course: AP®︎/College Calculus AB > Unit 9

Lesson 2: AP Calculus AB 2015 free response- 2015 AP Calculus AB/BC 1ab
- 2015 AP Calculus AB/BC 1c
- 2015 AP Calculus AB/BC 1d
- 2015 AP Calculus AB 2a
- 2015 AP Calculus AB 2b
- 2015 AP Calculus 2c
- 2015 AP Calculus AB/BC 3a
- 2015 AP Calculus AB/BC 3b
- 2015 AP Calculus AB/BC 3cd
- 2015 AP Calculus AB/BC 4ab
- 2015 AP Calculus AB/BC 4cd
- 2015 AP Calculus AB 5a
- 2015 AP Calculus AB 5b
- 2015 AP Calculus AB 5c
- 2015 AP Calculus AB 5d
- 2015 AP Calculus AB 6a
- 2015 AP Calculus AB 6b
- 2015 AP Calculus AB 6c

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# 2015 AP Calculus AB 6b

Point on curve where tangent is vertical.

## Want to join the conversation?

- can a tangent touch a curve twice?(1 vote)
- It is possible for two (or more) points on a curve to have the same tangent line, yes.

For example, the line 𝑦 = 0.5𝑥 + 3 is tangent to

the curve 𝑦 = 𝑥⁴ + 2𝑥³ − 3𝑥² − 3.5𝑥 + 7

in both the points (−2, 2) and (1, 3.5)(2 votes)

- Can anyone point to a video in the playlist where Sal goes thru this procedure? I've been through most of the AP/AB playlist and to the best of my recollection, this is the first time I've seen that we can flip the dy/dx to dx/dy. It's cool that we can do this; but I don't remember seeing this earlier. Thanks for any help!(1 vote)
- How come (3,2) isn't an answer?(0 votes)
- Because dy/dx at that point is 2/9. That point is on the curve, but it has a different slope than (3, -1)(2 votes)

## Video transcript

- [Voiceover] "Find the
coordinates of all points "on the curve at which the line tangent "to the curve at that point is vertical." So we wanna figure out
the points on that curve where the tangent line is vertical. So let's just remind
ourselves what the slope of a tangent line is, or what it isn't. I guess this may be a better
way to think about it. If you have a horizontal line, so let me draw a horizontal line. If you have a horizontal line like that, well then your slope is zero. Or you can say your rate of change of y with respect to x is equal to zero. What about a vertical line? If you have a vertical line like that, what is your rate of change
of y with respect to x? Well, some people might say it's infinity, or you could say it's undefined. It's undefined in some way because at some point your rate, or one way to think about it,
you're gonna try to divide by zero because you
have a huge change in y over no change in x. Or another way to think about it that's a little bit more in line with that is you could say that your
change in x with respect to change in y, notice I
took took the reciprocal, so now we're talking about
a change at the derivative of x with respect to y is equal to zero because your y can change,
but as your y changes, your x does not change. So can we use this little
insight here on vertical lines to think about the coordinates
of all points in the curve at which the line tangent to the curve at that point is vertical. Well before, they told us, they gave us the curve of the equation, and they also told us what dy, dx is. Let me just rewrite 'em again
just so we have 'em there. So we know that y to the third minus xy is equal to two. This is the equation of our curve. And we know that dy, dx is equal to y over 3y squared minus x. So one thing we could do is, well, let's just figure out
what the derivative of x with respect to y is and set
that to be equal to zero. So this is the derivative
of y with respect to x. If we take the reciprocal of that, the derivative of x with respect to y, it's just the reciprocal of this, is going to be the reciprocal
of what we have here. So it's going to be 3y squared minus x over y. And if we want this to be equal to zero, like we said right over here, well then that's only going to happen if the numerator is equal to zero. So we could say, okay, at what x, y pair does this numerator equal to zero? 3y squared minus x is equal to zero. Can add x to both sides
and you get 3y squared is equal to x. Now, another way you could
have thought about it is, well, at what x and y
values does the derivative of y with respect to x become undefined? Well, that's gonna become undefined if the denominator here is zero. But when you're dealing
with things like undefined, it gets a little bit more hand-wavy. I like to just think of
this as the rate at which x is changing with respect to y is zero. And so that got us to the same conclusion. Well, for that to be true, x has to be equal to 3y squared. And of course, the x, y
pair has to also satisfy the equation for the curve. So why don't we use both
of these constraints, and then we can solve for x and y? And so the easiest thing
I can think of doing is let's substitute x with 3y squared because they are the same, that's the second constraint. So if we take our original
equation of the curve, we get y to the third minus, instead of writing x, I could 3y squared. 3y squared times y is equal to two. And so we get y to the third minus 3y to the third is equal to two. This is negative 2y to the third is equal to two. Can divide both sides by negative two and we get y to the third is equal to negative one, or y is equal to negative one. Negative one to the third
power is negative one. Now if y is negative one, what is x? Well, x is going to be equal to three times negative one squared. So negative one squared is just one, so x is going to be equal to three. So the point on that curve
at which the tangent line is vertical is going to be the point three, comma, negative one. And we are all done.