New to Integrating, I need a lot of help

**R3ap3r** · April 26th 2008, 02:02 PM

I need a lot of help understanding this as I am completely lost at what to do
Evaluate to 4 significant digits: $\int_{-1} ^5\;\frac{5x^2}{\sqrt{x^3+3}}\;dx$

**Mathstud28** · April 26th 2008, 02:07 PM

Originally Posted by R3ap3r

I need a lot of help understanding this as I am completely lost at what to do
Evaluate to 4 significant digits: $\int_{-1} ^5\;\frac{5x^2}{\sqrt{x^3+3}}\;dx$

the derivative of the inside ofthe quanityt is $\frac{3}{5}$ of whats on the outside

**icemanfan** · April 26th 2008, 02:08 PM

Originally Posted by R3ap3r

I need a lot of help understanding this as I am completely lost at what to do
Evaluate to 4 significant digits: $\int_{-1} ^5\;\frac{5x^2}{\sqrt{x^3+3}}\;dx$

You can actually find an exact value for this integral. Use the fact that the derivative of $\sqrt{x^3 + 3}$ is $\frac{3x^2}{2 \sqrt{x^3 + 3}}$ .

**R3ap3r** · April 26th 2008, 02:13 PM

Originally Posted by Mathstud28

the derivative of the inside ofthe quanityt is $\frac{3}{5}$ of whats on the outside

I'm going to need a little bit more help than that. Step by step process if possible as I have a lot of these to do and one step by step process will teach me a lot.

**icemanfan** · April 26th 2008, 02:20 PM

Originally Posted by R3ap3r

I need a lot of help understanding this as I am completely lost at what to do
Evaluate to 4 significant digits: $\int_{-1} ^5\;\frac{5x^2}{\sqrt{x^3+3}}\;dx$

Ok, first step is to do a little rearranging of constants inside and outside the integral. Since you can pull out a constant, begin with

$\frac{10}{3}\int_{-1} ^5\;\frac{3x^2}{2\sqrt{x^3+3}}\;dx$ .

Then you have inside the integral the derivative of $\sqrt{x^3 + 3}$ , so you can evaluate to $\frac{10}{3}(\sqrt{5^3 + 3}-\sqrt{(-1)^3 + 3})$ .

**R3ap3r** · April 26th 2008, 02:28 PM

Originally Posted by icemanfan

Ok, first step is to do a little rearranging of constants inside and outside the integral. Since you can pull out a constant, begin with

$\frac{10}{3}\int_{-1} ^5\;\frac{3x^2}{2\sqrt{x^3+3}}\;dx$ .

Then you have inside the integral the derivative of $\sqrt{x^3 + 3}$ , so you can evaluate to $\frac{10}{3}(\sqrt{5^3 + 3}-\sqrt{(-1)^3 + 3})$ .

alright good start, but again i'm new to all this so how exactly did you rearrange it like that? I know you said pull out a constant but where did you get that from?

**icemanfan** · April 26th 2008, 02:34 PM

Originally Posted by R3ap3r

alright good start, but again i'm new to all this so how exactly did you rearrange it like that? I know you said pull out a constant but where did you get that from?

Originally, you had the constant of 5 in the numerator. I rearranged as $5 = \frac{10}{3}*\frac{3}{2}$ because I wanted to get $3x^2$ in the numerator, as $3x^2$ is the derivative of $x^3 + 3$ .

**icemanfan** · April 26th 2008, 02:40 PM

Originally Posted by R3ap3r

I need a lot of help understanding this as I am completely lost at what to do
Evaluate to 4 significant digits: $\int_{-1} ^5\;\frac{5x^2}{\sqrt{x^3+3}}\;dx$

It might help to view this as $\int_{-1} ^5\;{5x^2}(x^3+3)^{\frac{-1}{2}}\;dx$

Solving integrals a lot of times means being aware of the form $\int_{a}^{b}\; u^n du$ where in this case $u = x^3 + 3, n = \frac{-1}{2}, du = 3x^2 dx$

**R3ap3r** · April 26th 2008, 02:41 PM

Ok I understand that whats the next step after that?

**icemanfan** · April 26th 2008, 02:46 PM

Originally Posted by R3ap3r

Ok I understand that whats the next step after that?

If you have an integral in the form $\int_{a}^{b} u^n du$ then its evaluation is $\frac{1}{n+1}b^{n+1} - \frac{1}{n+1}a^{n+1}$ .

**R3ap3r** · April 26th 2008, 03:03 PM

So what I'm getting from that is $\frac{1}{-\frac{1}{2} + 1}\; 5^{-\frac{1}{2} + 1} - \frac{1}{-\frac{1}{2} + 1}\; -1^{1\frac{1}{2} + 1}$

Is this right so far?

**icemanfan** · April 26th 2008, 03:36 PM

Originally Posted by R3ap3r

So what I'm getting from that is $\frac{1}{-\frac{1}{2} + 1}\; 5^{-\frac{1}{2} + 1} - \frac{1}{-\frac{1}{2} + 1}\; -1^{1\frac{1}{2} + 1}$

Is this right so far?

Don't forget what your u is.

**Jhevon** · April 26th 2008, 03:53 PM

Originally Posted by R3ap3r

I need a lot of help understanding this as I am completely lost at what to do
Evaluate to 4 significant digits: $\int_{-1} ^5\;\frac{5x^2}{\sqrt{x^3+3}}\;dx$

if you can't see what to multiply by to get everything in the proper form, it is ok. just solve for it.

Let $u = x^3 + 3$

$\Rightarrow du = 3x^2~dx$

$\Rightarrow \frac 13 ~du = x^2~dx$ ................(you could also have said $\frac 53~du = 5x^2 ~dx$ )

So now you just replace $x^2~dx$ with $\frac 13~du$ and you integral now becomes:

$\frac 53 \int \frac 1{\sqrt{u}}~du = \frac 53 \int u^{- 1/2}~du$

Alternatively, you could let $u^2 = x^3 + 3$ and that would result in the integral $\frac {10}3 \int 1 ~du$

**R3ap3r** · April 26th 2008, 04:08 PM

uh my answer i got is 6.472. Am I even in the ballpark?

**Babs0201** · April 26th 2008, 05:09 PM

I got 32.9983 as an answer. Hope that helps.

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