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# Mean value theorem example: square root function

AP.CALC:
FUN‑1 (EU)
,
FUN‑1.B (LO)
,
FUN‑1.B.1 (EK)

## Video transcript

- [Voiceover] Let f of x be equal to the square root of four x minus three, and let c be the number that satisfies the mean value theorem for f on the closed interval between one and three, or one is less than or equal to x is less than or equal to three. What is c? So let's just remind ourselves what it means for c to be the number that satisfies the mean value theorem for f. This means that over this interval, c is a point, x equals c is a point where the slope of the tangent line at x equals c, so I could write f prime of c, so that is the slope of the tangent line when x is equal to c, this is equal to the slope of the secant line that connects these two points. So this is going to be equal to, see the slope of the secant line that connects the points three, f of three, and one, f of one. So it's going to be f of three minus f of one over three minus one. And if you wanted to think about what this means visually, it would look something like this. So if this is our x axis and this is one, two, actually let me spread it out a little bit more. One, two, and three. And so you have one, comma, f of one, right over there, so that is at the point one, comma, f of one, and we could evaluate that, actually, that's one, comma, one. So that's gonna be the point one, comma, one. And then you have the point three, comma, let's see, you're gonna have four times three is 12 minus three is nine, so it's gonna be three, comma, three. So maybe it's right over there, three, comma, three, and the curve might look something like this, so it might look something like that, so if you think about the slope of the line that connects these two points, so this line that connects those two points, all the mean value theorem, I'm doing a different color. All the mean value theorem tells us is that there's a point between one and three where the slope of the tangent line has the exact same slope. So if I were to eyeball it, it looks like it's right around there, although we are actually going to solve for it. So, some point where the slope of the tangent line is equal to the slope of the line that connects these two end points and their corresponding function values. So that is c, that would be c right over there. So really we just have to solve this. So let's first just find out what f prime of x is, and then we could substitute a c in there and then we can evaluate this on the right hand side. So I'm gonna rewrite f of x, f of x is equal to, and I'm gonna write it as four x to the minus three to the one half power. Makes it a little bit more obvious that we can apply the power rule and the chain rule here. So f prime of x is going to be the derivative of four x minus three to the one half with respect to four x minus three, so that is going to be one half times four x minus three to the negative one half, and then we're gonna multiply that times the derivative of four x minus three with respect to x. Well, derivative of four x with respect to x is just four, and the derivative of negative three with respect to x, well that's just gonna be zero, so the derivative of four x minus three with respect to x is four. So times four. so f prime of x is equal to four times one half, which is two, over the square root of four x minus three. Four x minus three to the one half would just be the square root of four x minus three, but it's the negative one half, so we're gonna put it in the denominator right over here. And so f prime of c, we could rewrite this as two over the square root of four c minus three, and what is that going to be equal to? That is going to be equal to, let's see, f of three we already figured out is three, f of one we already figured out is one, and so we get three minus one over three minus one, well that's gonna be two over two, which is equal to one. So there's some point between one and three where the derivative at that point, the slope of the tangent line, is equal to one. So let's see if we can solve this thing right over here. We can multiply both sides of this by four c by the square root of four c minus three, and so then we are going to get two is equal to the square root of four c minus three. All I did is multiply both sides of this by square root of four c minus three to get rid of this in the denominator, and so let's see, now we get rid of the radical, we can square both sides, and so actually let me just show that, so now we can square both sides, so we get four is equal to four c minus three, add three to both sides, seven is equal to four c, and then divide both sides by four. I'll go right here to do it, you're going to get c is equal to seven fourths. C is equal to seven over four, which is equal to one and three fourths, or we could view this as 1.75. So actually, the c value is a little bit closer, I hand drew this, it's closer to about right over there on our diagram, actually that looks pretty close, that actually looks pretty good, I just hand drew this curve so it's definitely not exact. But anyway, hopefully that gives you a sense of what's going on here. We're just saying, hey, the mean value theorem gives us some c where the slope of the tangent line is the same as the slope of the line that connects one, f of one, and three, f of three.
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