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Math Help - Rearranging equation from graph

  1. #1
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    Rearranging equation from graph

    Hi,

    I am having some (major) problems trying to rearrange an equation which defines a line on a graph- which Excel produced. They are in the format y=(-ax^2)+bx. I am trying to rearrange the equation to give x=.....

    The two equations are as follows.

    y= -0.4615x^2+29.381x in other words y = (-0.4615*x^2) + (29.381*x)

    y= -0.7565x^2+41.689x in other words y= (-0.7565*x^2) + (41.689*x)

    Unfortunately, my ability does not extend beyond putting the brackets in, and despite numerous attempts, I cannot work out how to calculate x.

    If anyone is able to help, it would be fantastic!

    Thanks
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  2. #2
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    e^(i*pi)'s Avatar
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    Re: Rearranging equation from graph

    Quote Originally Posted by asdf51 View Post
    Hi,

    I am having some (major) problems trying to rearrange an equation which defines a line on a graph- which Excel produced. They are in the format y=(-ax^2)+bx. I am trying to rearrange the equation to give x=.....

    The two equations are as follows.

    y= -0.4615x^2+29.381x in other words y = (-0.4615*x^2) + (29.381*x)

    y= -0.7565x^2+41.689x in other words y= (-0.7565*x^2) + (41.689*x)

    Unfortunately, my ability does not extend beyond putting the brackets in, and despite numerous attempts, I cannot work out how to calculate x.

    If anyone is able to help, it would be fantastic!

    Thanks
    Take y from both sides: -0.4615x^2+29.381x-y = 0

    Use the quadratic formula to find x in terms of y. If you want to find the roots of the equation then y=0 and you can factor.


    Spoiler:

    x = \dfrac{-29.381 \pm \sqrt{29.381^2 + 4 \times 0.4615 \times y}}{2 \times -0.4615}
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  3. #3
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    Re: Rearranging equation from graph

    Quote Originally Posted by e^(i*pi) View Post
    Take y from both sides: -0.4615x^2+29.381x-y = 0

    Use the quadratic formula to find x in terms of y. If you want to find the roots of the equation then y=0 and you can factor.


    Spoiler:

    x = \dfrac{-29.381 \pm \sqrt{29.381^2 + 4 \times 0.4615 \times y}}{2 \times -0.4615}

    Thanks for the reply. However, I can't seem to get it to work.

    If y=-0.4615*(x^2)+29.381*x, and x=20, then y=403.02.

    However, using the equation you gave, if you use 403 as the y value, then I cannot get 20 as the value for x.

    Am I just being stupid here or what!?

    Thanks!
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  4. #4
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    Re: Rearranging equation from graph

    Just wondering if anyone is able to help here- I still cannot work it out!

    Thanks
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