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## Finding particular solutions using initial conditions and separation of variables

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# Particular solutions to differential equations: exponential function

AP Calc: FUN‑7 (EU), FUN‑7.E (LO), FUN‑7.E.1 (EK), FUN‑7.E.2 (EK), FUN‑7.E.3 (EK)

## Video transcript

- [Voiceover] We're told that F of seven is equal to 40 plus five,
E to the seventh power, and F prime of X is equal to five, E to the X. What is F of zero? So to evaluate F of zero, let's take the anti-derivative
of F prime of X, and then we're going to have a constant of integration there, so we can use the information
that they gave us up here that F of seven is equal to this. This might look like an expression. Well, it is an expression, but it's really just a number. There's no variables in this, and so we can use that to solve for our constant of integration, and then we will have fully known what F of X is, and we can use that to evaluate F of zero, so let's just do it. So if F prime of X is
equal to five, E to the X, then F of X is going to be equal to the anti-derivative of F prime of X, or the anti-derivative
of five, E to the X, DX, and this is the thing
that I always find amazing about exponentials, and actually, let me just take a step. I'll take that five out of the integral so it becomes a little bit more obvious. And so the anti-derivative of E to the X, well, that's just E to the X because the derivative of E to the X is E to the X, which I find amazing every time I have to manipulate
or take the derivative or anti-derivative of E to the X. So this is gonna be
five, E to the X, plus C, and you can verify. Take the derivative of
five, E to the X, plus C. The derivative of five, E to the X, well, that's five, E to the X, so that works out. Well, and the derivative of C is zero, so you wouldn't see it over here. So now, let's use this information to figure out what C is so that we know exactly what F of X is, and then we can evaluate F of zero. So we know that F of seven, so when X is equal to seven, this expression is going
to evaluate to this thing, 40 plus five, E to the seven. So, five times E to the seventh power plus C is equal to 40, plus five, E to the seventh power. And notice, all I did is say, okay, F of seven. Well, if this is F of X-- Let me write this down. So, if this is F of seven, if this is F of X, I just replaced the X with a seven to find F of seven, and we know that F of seven is also going to be equal to that. They gave us that information, but when you just look at this, it's pretty easy to figure
out what C is going to be. You can subtract five, E to the seven from both sides, and you see that C is equal to 40. And so we can rewrite F of X. We can say that F of X is equal to five, E to the X, plus C, which is 40. And so now, from that, we can evaluate F of zero. F of zero is going to be five times E to the zero power, plus 40. E to the zero is one, so it's gonna be five times one, which is just five, plus 40, which is equal to 45, and we're done.

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