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Math Help - Can anyone explain subsitiution to me please?

  1. #1
    Newbie
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    Sep 2009
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    Question Can anyone explain subsitiution to me please?

    I am really not understanding the substitution method. Can someone please explain it to me?

    Example:

    Y=4x-9
    y= x-3

    I would really appreciate it =]
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  2. #2
    Senior Member
    Joined
    Nov 2008
    Posts
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    Hello Lexifer.

    Quote Originally Posted by Lexifer View Post
    I am really not understanding the substitution method. Can someone please explain it to me?

    Example:

    Y=4x-9
    y= x-3

    I would really appreciate it =]
    You have to equations
    Y=4x-9 (eq1)
    y= x-3 (eq2)

    You know y because of (eq2) and so you substitute it in (eq1), this leads to

    x-3 = 4x-9 (eq2 in eq1)

    and you solve it for x.

    Back to the two equations
    Y=4x-9 (eq1)
    y= x-3 (eq2)

    You can use a substitution for x, too.
    solving (eq2) for x

    x = y+3 (eq2' )

    and substitute it in (eq1) for x

    4(y+3)-9=y (eq2 in eq1)

    [In (eq1) you write y+3 instead of x because of eq2' ]

    now you need to solve this equation for y

    Still any questions? Then let us know

    Yours
    Rapha
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