Changing the subject

**fallschirm2** · March 24th 2009, 02:59 PM

Hello,

Any help on this would be appreciated.

I need to alter the following formula to make "L" the subject of the formula.

$A=\dfrac{5WL^4}{384EI}$

Thank you in advance

Fallschirm2

**rtblue** · March 24th 2009, 03:40 PM

ok. First of all, i am puzzled how you make the equation look so perfect, I can tell that you are not typing this. If anyone can tell me how to make numbers, and variables look like that, it would be appreciated. Ok, on to the problem.

So, you have to isolate L.

First, Multiply the whole Equation by the denominator of the fraction on the left, and you recieve:

$384EIA=5WL^4$

next, divide by 5W, and you will recieve:

$\frac{384EIA}{5W}=L^4$

next. Make everything to the fourth root, so you cancel out that exponent 4. You get:

$\sqrt[4]{\frac{384EIA}{5W}}=L$
You cannot have any roots in the denominator. So, you multiply the denominator by 5W(to the fourth root) and you multiply 384EIA(to the fourth root) by 5W(to the fourth root). You get:

$\frac{\sqrt[4]{1920EIA}}{5W}=L$

Please excuse me if I made a mistake, or if the work isn't clear(which probably isn't). If someone can tell me how to get the thing that allows me to show my work neatly, that would be helpful.

**Chris L T521** · March 24th 2009, 03:54 PM

Originally Posted by rtblue

[snip]First of all, i am puzzled how you make the equation look so perfect, I can tell that you are not typing this. If anyone can tell me how to make numbers, and variables look like that, it would be appreciated.[/snip]

We use LaTeX here on MHF. You can find more about it in our LaTeX Help subforum.

**stapel** · March 24th 2009, 05:11 PM

Originally Posted by fallschirm2

$A=\dfrac{5WL^4}{384EI}$

$384EI\, =\, 5WL^4$

$\frac{384EI}{5W}\, =\, L^4$

$\sqrt[4]{\frac{384EI}{5W}}\, =\, L$

**rtblue** · March 24th 2009, 05:14 PM

well. The answer that has been given my stapel, would be counted wrong on an exam. As a general rule, radicals cannot be in the denominator of a fraction. They just can't. Numberator is ok for radicals, but the denominator isn't.

**JD-Styles** · March 24th 2009, 08:31 PM

Originally Posted by rtblue

well. The answer that has been given my stapel, would be counted wrong on an exam. As a general rule, radicals cannot be in the denominator of a fraction. They just can't. Numberator is ok for radicals, but the denominator isn't.

That all depends on the teacher now doesn't it? There's nothing mathematically wrong about having radicals in the denominator. Actually after about 10th grade math, I've never had anyone worry about radicals in the denominator.

**earboth** · March 24th 2009, 11:11 PM

Originally Posted by rtblue

... You get:

$\frac{\sqrt[4]{1920EIA}}{5W}=L$

...

Originally Posted by rtblue

well. The answer that has been given by rtblue, would be counted wrong on an exam. ...

$\dfrac{384EI}{5W}\, =\, L^4~\implies~|L|= \sqrt[4]{\dfrac{384EI}{5W}} = \dfrac{\sqrt[4]{1920EIAW}}{5W}$

**rtblue** · March 25th 2009, 03:49 PM

oh, i'm sorry

I guess i left out the W under the radical. my mistake.

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