부분적분법

This integrand is the product of two functions, with no obvious substitution. So, let's try integration by parts, with one part $e^x$ and one part $x^3$.

$$\begin{array}{c | l | l |} &&\\ \text{Option 1:\quad}& u=e^x &\mathrm{d}u=e^x ~\mathrm{d}x\\[10pt] ~& \mathrm{d}v=x^3~\mathrm{d}x&v=\frac{1}{4}x^4\\[10pt] &&\\ \text{Option 2:\quad}& u=x^3&\mathrm{d}u=3x^2~\mathrm{d}x\\[10pt] ~& \mathrm{d}v=e^x~ \mathrm{d}x&v=e^x\\[10pt] \end{array}$$

At first glance, multiplying the red functions and multiplying the blue functions give largely equivalent integrands to what we started with–none of them with obvious antiderivatives. In previous questions, we were able to choose $u=x$, and then $\mathrm{d}u=\mathrm{d}x$, so the “$x$" in the integrand effectively went away. Here, we see that choosing $u=x^3$ will lead to $\mathrm{d}u=3x^2\mathrm{d}x$, which has a lower power. If we repeatedly perform integration by parts, choosing $u$ to be the power of $x$ each time, then after a few iterations it should go away, because the third derivative of $x^3$ is a constant.

So, we start with Option 2: $u=x^3$, $\mathrm{d}v=e^x\mathrm{d}x$, $\mathrm{d}u=3x^2\mathrm{d}x$, and $v = e^x$.

$$\begin{aligned} \int x^3 e^x \mathrm{d}x&= \underbrace{x^3}_{u}\underbrace{e^x}_{v} - \int \underbrace{e^x}_v \cdot\underbrace{ 3x^2 \mathrm{d}x}_{\mathrm{d}u}\\ &= {x^3}{e^x} -3 \int {e^x} \cdot x^2 \mathrm{d}x \\ \text{Now, we take $u=x^2$ and $\mathrm{d}v=e^x\mathrm{d}x$, so $\mathrm{d}u=2x\mathrm{d}x$ and $v=e^x$. We're only using integration by parts on the actual integral--the rest of the function stays the way it is.} \\ &=x^3e^x -3 \left[\underbrace{x^2e^x}_{uv} - \int \underbrace{e^x}_{v} \cdot \underbrace{2x\mathrm{d}x}_{\mathrm{d}u}\right] \\ &=x^3e^x - 3x^2e^x + 6\int x e^x\mathrm{d}x \\ \text{Continuing, we take $u=x$ and $\mathrm{d}v=e^x\mathrm{d}x$, so $\mathrm{d}u=\mathrm{d}x$ and $v=e^x$. This is the step where the polynomial part of the integrand finally disappears.} \\ &=x^3e^x - 3x^2e^x + 6\left[\underbrace{xe^x}_{uv} - \int \underbrace{ e^x}_v \underbrace{\mathrm{d}x}_{\mathrm{d}u}\right]\\ &=x^3e^x-3x^2e^x+6xe^x - 6e^x+C \\ &=e^x\left(x^3-3x^2+6x - 6\right)+C \end{aligned}$$

Let's check that this makes sense: the derivative of $e^x\left(x^3-3x^2+6x - 6\right)+C$ should be $x^3e^x$. We differentiate using the product rule.

$$\begin{aligned} \frac{\mathrm{d}}{\mathrm{d}x}\left\{e^x\left(x^3-3x^2+6x - 6\right)+C\right\} &=e^x\left(x^3-3x^2+6x - 6\right)+e^x\left(3x^2-6x+6\right)\\ &=e^x\left(x^3-3x^2+3x^2+6x-6x-6+6\right)=x^3e^x \end{aligned}$$

Remark: In order to be technically correct in our antidifferentiation, we should add the $+C$ as soon as we do the first integration by parts. However, when we are using integration by parts, we usually end up evaluating an integral at the end, and we add the $+C$ at that point. Since the $+C$ comes up eventually, it is common practice to not clutter our calculations with it until the end.