3
$\begingroup$

Would appreciate it if someone would please help me solve this

$\int y\;\ln y\, \mathbb dy$

taking time to explain reason for each step taken.

Thanks in advance!

  • 0
    No you don't necessarily need integration by parts, if you are aware of$a$simple formula. Please see my answer below.2011-07-10

4 Answers 4

17

Use integration by parts:

$ \int \frac{df}{dy}\ g \ \ \text{d}y = fg - \int f \ \frac{dg}{dy} \ \ \text{d}y $

Thus we get

$ \int \frac{d (y^2/2)}{dy}\ \ln y \ \ \text{d}y = \frac{y^2}{2}\ \ln y - \int \frac{y^2}{2} \frac{d \ln y}{dy} \ \ \text{d}y$

Now $\displaystyle \frac{d \ln y}{dy} = \frac{1}{y}$.

The last integral becomes

$\int \frac{y^2}{2} \frac{1}{y} \ \ \text{d}y = \int \frac{y}{2} \ \ \text{d}y = \frac{y^2}{4}$

Thus your integral is

$ \frac{y^2 \ln y}{2} - \frac{y^2}{4} + C$

The reason to use integration by parts is that the derivative of $\displaystyle \ln y$ is $\displaystyle \frac{1}{y}$ which cancels out with one of the $y$ got from the other term.

For instance try the same technique with the following:

$\int y^{1000} \ln y \ \text{d}y$

  • 0
    My \$0.02 based on teaching freshman calculus is that I think it's a bad idea for instructors to leave out the dx or dy in an integral when teaching beginning students.2011-07-10
6

We go for integration by parts, since we are looking at the product of two unrelated functions, and there is no obvious substitution. I do not know what notation you use when setting up an integration by parts, so I will guess. The integration by parts "formula" can be written in two completely equivalent ways, apart from the choice of names. These are $\int u\,dv=uv -\int v\,du\qquad\text{and}$ $\int v\,du=uv -\int u\,dv.$ This formula comes directly from the Product Rule for derivatives. Please consult your text for details. In integration by parts, often the key question is: what shall I choose as $u$ (and what shall I choose as $dv$).

Maybe we should let $u=y$. Then $dv$ must be $\ln y \,dy$. That sounds good, we will then have $du=dy$, which is a simplification. Great. We have let $dv=\ln y\,dy$, so we need to find $v$, that is, to integrate $\ln y$. Looks a little ugly, or at least not immediate.

Let's not despair, maybe the integral of $\ln y$ will not be too hard. But we might as well take a side glance at the other plausible choice, $du=y\,dy$, $v=\ln y$. Then $u=y^2/2$ (we don't worry about constants of integration yet). Ouch, looks worse than $y$! And what looks worse usually is worse. But let's persist: $dv=(1/y)\,dy$. That's really good, ugly stuff is gone, we are basically finished.

So, formally now, let $du=y\,dy$, let $v=\ln y$. Then $u=y^2/2$, $dv=(1/y)\,dy$. (Do lay it out more nicely than I have.) So we have $\int y\ln y \,dy=uv-\int u\,dv=\frac{y^2}{2}\ln y-\int \frac{y}{2}dy.$

Integrate, remembering now about the constant of integration. $\int y\ln y\,dy=\frac{y^2\ln y}{2}-\frac{y^2}{4}+C.$

Comment: When we integrate $x^3e^{2x}$, for example, the integration by parts strategy focuses on lowering the power of $x$. That is "usually" a good strategy. Exceptions to this general heuristic are things like $x^3\ln x$, and $x^3 \arctan x$.

Extreme examples are $\int \ln x\,dx$, $\int \arctan x\,dx$, and $\int \arcsin x\, dx$, where it looks as if we don't even have a "product". But if, for example in $\int \ln x \,dx$, we let $du=dx$, $v=\ln x$, everything works out quickly.

Added: Could substitution be a feasible approach? It seems sensible to try for $z=\ln y$, or equivalently $y=e^z$. Then $dy=e^z\,dz$. Our integral becomes $\int ze^{2z}\,dz.$ This is an integration by parts, so the substitution didn't buy us much! But it's a more "normal" integration by parts. The rest is routine.

5

Here is a method of doing this by substitution. Put $y=e^{x}$. Then you have $dy = e^{x} \ dx$. Substituting we have

\begin{align*} \int e^{2x} \cdot x \ dx &= \frac{1}{4}\int e^{t} \cdot t\ dt \qquad \Bigl[ \text{substituting} \ t = 2x \Bigr] \\ &= \frac{1}{4}\int e^{t} \cdot \bigl( t + 1 \bigr) \ dt - \frac{1}{4}\int e^{t} \ dt \\ &= \frac{1}{4}e^{t} \cdot t -\frac{1}{4} e^{t} + C \qquad\qquad\qquad \Bigl[ \because \small \int e^{x} \cdot \Bigl( f(x) + f'(x) \Bigr) \ dx = e^{x} \cdot f(x) + C \ \Bigr] \\ &=\frac{1}{4}\cdot e^{2x} \cdot 2x - \frac{1}{4}\cdot e^{2x} +C \\ &=\log{y} \cdot y^{2} \times \frac{1}{2} - \frac{1}{4} \cdot y^{2} +C \end{align*}

  • 0
    @Martin: I think people coming here are matured enough to know when $\{$ denotes a fractional part and when $\{\}$ denotes a bracket operator.2011-07-10
2

Here's another way to arrive at the result (which also provides a partial intuition). Fix $0 < y \leq 1$. Then, $ \int_0^y {u\ln u \,du} = \int_0^y {u\bigg( - \int_u^1 {\frac{1}{v}} \,dv\bigg)du} = \int_{u = 0}^y {\bigg(\int_{v = u}^1 {\frac{{ - u}}{v}\,dv\bigg)du} } . $ Interchanging the order of integration (as usual, it is helpful to draw a picture) yields $ \int_0^y {u\ln u \,du} = \int_{v = 0}^y {\bigg(\int_{u = 0}^v {\frac{{ - u}}{v}\,du\bigg)dv} } + \int_{v = y}^1 {\bigg(\int_{u = 0}^y {\frac{{ - u}}{v}\,du} \bigg)dv} . $ Hence $ \int_0^y {u\ln u \,du} = \int_{v = 0}^y {\frac{1}{v}} \frac{{ - v^2 }}{2}\,dv + \int_{v = y}^1 {\frac{1}{v}\frac{{ - y^2 }}{2}\,dv} = - \frac{{y^2 }}{4} + \frac{{y^2 \ln y}}{2}. $