Results 1 to 8 of 8

Math Help - Solve By Factoring Help Need

  1. #1
    Newbie
    Joined
    May 2010
    Posts
    6

    Solve By Factoring Help Need

    height of ball thrown = h
    t = time in seconds

    h = -4.9t^2 + 38t + 1.75

    what is the time of the ball when h = 50

    I need to be able to solve it by factoring? i can do it if the numbers 4.9 and 38 have a common denominator but can not do when they are decimals?
    any help appreciated
    Last edited by Ashley911; May 16th 2010 at 08:59 AM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    -1
    e^(i*pi)'s Avatar
    Joined
    Feb 2009
    From
    West Midlands, England
    Posts
    3,053
    Thanks
    1
    Quote Originally Posted by Ashley911 View Post
    height of ball thrown = h
    t = time in seconds

    h = -4.9t^2 + 38t + 1.75

    what is the height of the ball after 3 seconds

    I need to be able to solve it by factoring? i can do it if the numbers 4.9 and 38 have a common denominator but can do when they are decimals?
    any help appreciated
    h(t) = -4.9t^2 + 38t + 1.75

    To find the height at 3 seconds solve h(3) by putting in 3 wherever you find t in the original equation. No factoring required

    h(3) = -4.9(3^2) + 38(3) + 1.75
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    May 2010
    Posts
    6
    I have to solve it by factoring its a homework question.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    May 2010
    Posts
    23
    I suppose you could plug in 50 for h, then subtract 50 from both sides

    should look something like
    -4.9t^2 + 38t - 48.25 = 0

    Then just use the quadratic formula to find the two roots of this equation. Using those two roots, you can find the factors of the polynomial. One of those roots will be your time, (the positive one).

    The quadratic formula will find your factors, however this is a bit redundant, since to find the factors you will need to find the roots, and one of the roots is your answer.

    Otherwise you could use the decomposition method shown here: How to Factor Second Degree Polynomials (Quadratic Equations) - wikiHow
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    May 2010
    Posts
    6
    Thanks but am still stuck here is my issue

    I need to find the two numbers that when multiplied = 4.9*48.25
    and
    when added together are = 48.25

    I keep getting decimals and believe this should not be the case.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    -1
    e^(i*pi)'s Avatar
    Joined
    Feb 2009
    From
    West Midlands, England
    Posts
    3,053
    Thanks
    1
    Quote Originally Posted by Ashley911 View Post
    Thanks but am still stuck here is my issue

    I need to find the two numbers that when multiplied = 4.9*48.25
    and
    when added together are = 48.25

    I keep getting decimals and believe this should not be the case.
    You won't get neat factors because b^2-4ac is not a perfect square
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Member rtblue's Avatar
    Joined
    Mar 2009
    From
    Birmingham, Alabama.
    Posts
    221
    h = -4.9t^2 + 38t + 1.75

    Use the quadratic formula

    \frac{\-b\pm\sqrt{b^2-4ac}}{2a}

    plug in the values to get

    \frac{-38\pm\sqrt{38^2-4(-4.9)(1.75)}}{2(-4.90)}

    Now solve for both values: + and -

    Once you have your roots then, then go ahead and solve.
    Follow Math Help Forum on Facebook and Google+

  8. #8
    -1
    e^(i*pi)'s Avatar
    Joined
    Feb 2009
    From
    West Midlands, England
    Posts
    3,053
    Thanks
    1
    The OP could also complete the square - it's a kind of factoring

    50 = -4.9t^2 + 38t + 1.75

    4.9t^2-38t = -48.25

    t^2 - \frac{38}{4.9} = -\frac{48.25}{4.9}

    \left(t- \frac{38}{9.8}\right)^2 - \left(\frac{38}{4.9}\right)^2 = -\frac{48.25}{4.9}

    \left(t- \frac{38}{9.8}\right)^2 = \left(\frac{38}{4.9}\right)^2 -\frac{48.25}{4.9}

    t - \frac{38}{9.8} = \pm \sqrt{ \left(\frac{38}{4.9} \right)^2 -\frac{48.25}{4.9}}


    t = \frac{38}{9.8} \pm \sqrt{ \left(\frac{38}{4.9} \right)^2 -\frac{48.25}{4.9}}
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. solve by factoring
    Posted in the Algebra Forum
    Replies: 1
    Last Post: August 2nd 2010, 05:16 AM
  2. Solve by factoring
    Posted in the Algebra Forum
    Replies: 2
    Last Post: March 24th 2009, 11:19 AM
  3. solve by factoring
    Posted in the Algebra Forum
    Replies: 2
    Last Post: January 24th 2009, 11:49 AM
  4. Help with factoring to solve for x
    Posted in the Algebra Forum
    Replies: 2
    Last Post: October 14th 2008, 04:58 PM
  5. Solve by factoring
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: April 2nd 2008, 05:49 PM

Search Tags


/mathhelpforum @mathhelpforum