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Math Help - help with find values

  1. #1
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    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.

    please help

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

    thank you
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  2. #2
    MHF Contributor
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    Talking

    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!
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  3. #3
    A riddle wrapped in an enigma
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    Quote Originally Posted by Mathaticaln00b View Post
    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.

    please help

    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
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  4. #4
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    Quote Originally Posted by masters View Post
    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?

    theres so much i dont understand please help me.

    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
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  5. #5
    MHF Contributor

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    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?
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