yego.me
💡 Stop wasting time. Read Youtube instead of watch. Download Chrome Extension

Negative definite integrals | Integration and accumulation of change | AP Calculus AB | Khan Academy


4m read
·Nov 11, 2024

We've already thought about what a definite integral means. If I'm taking the definite integral from ( a ) to ( b ) of ( f(x) , dx ), I can just view that as the area below my function ( f ).

So, if this is my y-axis, this is my x-axis, and ( y ) is equal to ( f(x) ). Something like that: ( y ) is equal to ( f(x) ). And if this is ( a ), and if this is ( b ), I could just view this expression as being equal to this area.

But what if my function was not above the x-axis? What if it was below the x-axis? So, these are going to be equivalent. Let's say, let me just draw that scenario. So let me draw a scenario where it's my x-axis that is my y-axis. And let's say I have, let's say I have a function that looks like that.

So that is ( y = g(x) ), and let's say that this right over here is ( a ), and this right over here is ( b ). And let's say that this area right over here is equal to 5. Well, if I were to ask you, what is the definite integral from ( a ) to ( b ) of ( g(x) , dx )? What do you think it is going to be?

Well, you might be tempted to say, "Hey, well it's just the area again between my curve and the x-axis." You might be tempted to say, "Hey, this is just going to be equal to 5." But you have to be very careful because if you're looking at the area above your curve and below your x-axis versus below your curve and above the x-axis, this definite integral is actually going to be the negative of the area.

Now, we'll see later on why this will work out nicely with a whole set of integration properties. But if you want to get some intuition for it, let's just think about velocity versus time graphs.

So if I... in my horizontal axis, that is time. My vertical axis, this is velocity, and velocity is going to be measured in meters per second. Time is going to be measured in seconds. Time is measured in seconds, and actually I'm going to do two scenarios here.

So let's say that I have a first velocity-time graph. Let's just call it ( v_1(t) ) which is equal to 3, and it would be 3 meters per second. So one, two, three. So it would look like that: that is ( v_1(t) ). And if I were to look at the definite integral going from time equals 1 to time equals 5 of ( v_1(t) , dt ), what would this be equal to?

Well, here my function is above my ( t )-axis, so I'll just go from 1 to 5, which will be around there. I could just think about the area here, and this area is pretty easy to calculate. It's going to be 3 meters per second times 4 seconds; that's my change in time.

So this is going to be 12 meters, and so this is going to be equal to 12. One way to conceptualize this is this gives us our change in position. If my velocity is 3 meters per second and since it's positive, you can conceptualize that as it's going to the right at 3 meters per second.

What is my change in position? Well, I would have gone 12 meters to the right, and you don't need calculus to figure that out. ( 3 , \text{m/s} \times 4 , \text{s} ) would be 12 meters. But what if it were the other way around? What if I had another velocity function?

Let's call that ( v_2(t) ) that is equal to negative 2 meters per second, and it's just a constant negative 2 meters per second. So this is ( v_2(t) ) right over here. What would or what should the definite integral from 1 to 5 of ( v_2(t) , dt ) be equal to?

Well, it should be equal to my change in position. But if my velocity is negative, that means I'm moving to the left. That means my change in position should be to the left as opposed to to the right.

So we can just look at this area right over here. When if you just look at it as the rectangle, it is going to be ( 2 \times 4 ), which is equal to 8. But you have to be very careful since it is below my horizontal axis and above my function. This is going to be negative.

And this should make a lot of sense. If I'm going 2 meters per second to the left for four seconds, or another way to think about it, if I'm going negative two meters per second for four seconds, then my change in position is going to be negative eight meters. I would have moved eight meters to the left if we say the convention is negative means to the left.

So the big takeaway is if it's below your function and above the horizontal axis, the definite integral, and if your ( a ) is less than ( b ), then your definite integral is going to be positive. If your ( a ) is less than ( b ), but your function over that interval is below the horizontal axis, then your definite integral is going to be negative.

And in the future, we'll also look at definite integrals that are a mix of both, but that's a little bit more complicated.

More Articles

View All
Mixed number subtraction
Let’s say that we want to figure out what is 7 and 11⁄12 minus 1 and 6⁄12. Pause this video and see if you can figure that out. All right, now let’s work on this together. So there’s a couple of ways that you could approach this. You can view this as the…
Darren's Great Big Camera - Smarter Every Day 21
Today on Smarter Every Day, you’re gonna learn about big rockets and big cameras. Is it going now? Woah! [Rushing air] Woohoo! Yeah! Oh! Hey, it’s me, Destin. I’m at the U.S. Space & Rocket Center with my new friend Darren, who’s got a great big camer…
Hurricane Katrina Survivor Gives Tours of Its Destruction | National Geographic
Let me tell you a little bit about the City of New Orleans. Right after Katrina, I kept hearing everybody say, “Why should we pay our tax dollars to bring New Orleans back? They below sea level.” I am a tour guide. I do Katrina tours. I never was an emoti…
Great White Sharks of Guadalupe Island | Most Wanted Sharks
NARRATOR: But everyone loves Lucy. The story of this great white is the classic “Finding Nemo” tale, but about 2,000 pounds heavier. When divers spotted Lucy back in 2008, her distinctive tail wound looked fresh. And she seemed in desperate need of a good…
LearnStorm Growth Mindset: Khan Academy's humanities content creator on social belonging
Hey, I’m Kim Kutz Elliott and I work on humanities content at Khan Academy. So yeah, I thought about things that were really difficult for me. One thing, um, that was hard for me was class discussion because I went to this history class, and I swear that…
They Shut Down My Coffee Company
What’s up guys, it’s Graham here. So, I think it’s about time that we have an open talk about what happened with my coffee company because I read all the comments asking what’s going on. Well, trying to get any update from me on the status of when it’s go…