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I am trying to evaluate the integral

$x-2|x|$ from -1 to 2

I know that this should give me

$x^2 /2 - x^2$ for the antiderivative

I then evaluate at 2 which gives me

$2 - 4 = -2$

Then evaluate at -1 $\frac{1}{2} - 1 = -\frac{1}{2}$

Then I find the difference

$-2 - \frac{1}{2} = -2.5$

This is wrong and I do not know why.

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2 Answers

up vote 4 down vote accepted

You should consider splitting the integral at the point where $|x|$ changes from $-x$ to $+x$, namely at $x=0$. So $$ \int_{-1}^2(x-2|x|)dx = \int_{-1}^0(x-2(-x))dx + \int_0^{2}(x-2x)dx $$ $$ =3\int_{-1}^0 xdx -\int_0^2xdx $$

which are integrals you should be able to evaluate.

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I dont follow what happend to the 0 to 2 (x-2x)dx part, where did the 3 go? –  user138246 Apr 22 '12 at 3:15
    
In the range $-1 \leq x \leq 0$ we have that $|x| = -x$, so I substitute $(-x)$ where $|x|$ was in the first integral. Then $x-2(-x) = 3x$ which is where $3\int_{-1}^0xdx$ comes from. Similarly, in the range $0 \leq x \leq 2$ we have that $|x| = x$, so I replace $|x|$ by $x$ in the second integral. Then $x-2x = -x$, which is where $-\int_0^2xdx$ comes from. –  nullUser Apr 22 '12 at 3:20
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Hint

$$|x|=\begin{cases} -x, &x \le 0\\x. &x \ge 0\end{cases}$$

So, $$\begin{align}\int_{-1}^2|x|\; \mathrm{d} x&=\int_{-1}^0|x| \;\mathrm{d}x+\int_0^2|x| \;\mathrm{d}x\\&=?\end{align}$$

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I do not know what that really means, x will always be positive. –  user138246 Apr 22 '12 at 3:12
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No, @Jordan. $x$ is negative when $-1 \le x \lt 0$. This is the reason why the integral of $|x|$ is different from $x$... –  user21436 Apr 22 '12 at 3:14
    
I thought absolute value meant that the value of that number is always positive. –  user138246 Apr 22 '12 at 3:15
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Will convince you on chat @Jordan –  user21436 Apr 22 '12 at 3:17
    
Jordan, you should read this answer a few times every day until you can explain every word to (say) a 7-year-old. Of course, Kanna only covers the absolute value part of the integral, but that's the only part you're struggling with. –  The Chaz 2.0 Apr 22 '12 at 3:45
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