# Math Help - help with find values

1. ## help with find values

I have a simple yet frustrating problem.

V=15

h= 2.54

r= ?

V= pi*h(3r^2 + h^2) / 6

what is the value of r?

Now I need a complete break down of the reasons for the working of this equation.

I have asked sevral sites and the explanations are difficult to understand.

I need it in plain english.

Someone said I had to divide V/(3*pi*h) to find r

now what i want to know is what formula he used to come to that decision telling me the answer like that is no good.

what i need is to know why make descisionlike this so i can solve ALL algrebra question.

I am looking for somone who will also perhaps be will to PM so i can understand

thank you

2. You are given a formula with variables V, h, and r.

You are given values for V and h.

You are asked for the corresponding value of r.

Plug the number for V in for V in the formula..

Plug the number for h in for h in the formula.

Simplify as much as you can.

Solve the resulting quadratic equation for the value of r.

If you get stuck or do not understand a step, please reply showing your progress and clearly stating where you are getting confused. Thank you!

3. Originally Posted by Mathaticaln00b
I have a simple yet frustrating problem.

V=15

h= 2.54

r= ?

V= pi*h(3r^2 + h^2) / 6

what is the value of r?

Now I need a complete break down of the reasons for the working of this equation.

I have asked sevral sites and the explanations are difficult to understand.

I need it in plain english.

Someone said I had to divide V/(3*pi*h) to find r

now what i want to know is what formula he used to come to that decision telling me the answer like that is no good.

what i need is to know why make descisionlike this so i can solve ALL algrebra question.

I am looking for somone who will also perhaps be will to PM so i can understand

thank you
Hi Mathaticaln00b,

$V=\frac{\pi h(3r^2+h^2)}{6}$

Substitute what you know...V=15, h=2.54...

$15=\frac{\pi(2.54)(3r^2+(2.54)^2)}{6}$

Apply the distributive property...

$15=\frac{7.62 \pi r^2+16.387064 \pi}{6}$

Clear the fraction (Multiply both sides by 6)...

$90=7.62 \pi r^2+16.387064$

Subtract 16.387064 from both sides...

$73.612936=7.62\pi r^2$

Divide both sides by $7.62 \pi$...

$\frac{73.612936}{7.62\pi}=r^2$

Take the square root of both sides...

$\sqrt{\frac{73.612936}{7.62\pi}}=r$

4. Originally Posted by masters
Hi Mathaticaln00b,

$V=\frac{\pi h(3r^2+h^2)}{6}$

Substitute what you know...V=15, h=2.54...

$15=\frac{\pi(2.54)(3r^2+(2.54)^2)}{6}$

Apply the distributive property...

$15=\frac{7.62 \pi r^2+16.387064 \pi}{6}$

Clear the fraction (Multiply both sides by 6)...

$90=7.62 \pi r^2+16.387064$

Subtract 16.387064 from both sides...

$73.612936=7.62\pi r^2$

Divide both sides by $7.62 \pi$...

$\frac{73.612936}{7.62\pi}=r^2$

Take the square root of both sides...

$\sqrt{\frac{73.612936}{7.62\pi}}=r$
hi thanks to both of you for answering my post.

I have a few questions though since i still dont know what r value is.

please tell me if i am supposed to know or not.

I figured out to a point that substition of values makes sense but to be honest i dont understand the rest of it.

also when using pi in the equation am i using the number 3.14 or not?

what i did was

v=3.14 x 2.54(3 x 3.14 x 2.54 x 2) + (3.14 x 2.54 x 2.54 x 2.54) / 6

then i x that by 3 and squared it

then i did 2.54 squared

and then divided by 6

i know this is wrong but im so confused

5. You said you were solving for r. You have a "v" in your final equation but no "r". what happened to it?

You wrote "v=3.14 x 2.54(3 x 3.14 x 2.54 x 2) + (3.14 x 2.54 x 2.54 x 2.54) / 6"

Okay, that first "3.14 x 2.54" is pi h but then the first thing inside your parentheses should be "3r^2". Instead you appear to have pi h times 2. Why? The second term inside the parenthes should be h^2 instead you have a separate parentheses and pi h^3. That, at least is correct: pi h (h^2)= pi h^3. Even assuming that the "x 2" should be "^2", your first term is pi h(3 pi h^2) instead of 3r^2. Are you just putting numbers together at random? Why not do what was suggested in the first response?