# Thread: Stuck on HW problem, can't see how I'm wrong

1. ## Stuck on HW problem, can't see how I'm wrong

Hello all,

The problem is this:

=

I worked it out like this:

$x^4/4 = 2b^4/4 - b^4/4$

But when I enter that in, they say I am wrong. Does anybody see what I'm doing wrong on this?

2. Originally Posted by JohnJames
Hello all,

The problem is this:

=

I worked it out like this:

$x^4/4 = 2b^4/4 - b^4/4$

But when I enter that in, they say I am wrong. Does anybody see what I'm doing wrong on this?
You have to use brackets in the last line:

$\left[\dfrac{x^4}4\right]^{2b}_b = \dfrac{(2b)^4}4-\dfrac{b^4}4 = \dfrac{15b^4}4$

3. Originally Posted by JohnJames
Hello all,

The problem is this:

=

I worked it out like this:

$x^4/4 = 2b^4/4 - b^4/4$

But when I enter that in, they say I am wrong. Does anybody see what I'm doing wrong on this?
$\int_{b}^{2b}x^{3}dx=\left[ \frac{x^{4}}{4}+c\right]^{2b}_{b}=\frac{(2b)^{4}}{4}+c-(\frac{b^{4}}{4}+c)=4b^{4}-\frac{b^{4}}{4}=\frac{15b^{4}}{4}$

4. Oh Wow, simple math got me this time lol, thanks to earboth and coscos