algebra

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how do you use the substitution method to solve the linear system?
-2x-5y=7
7x=y=-8

  • algebra -

    Is the second line supposed to be 7x + y = -8, or 7x - y = -8? You sweem to have inserted a second equal sign by mistake.

    Whichever it is, use the second equation to derive an equation for y in terms of x. Then substitute that "y" value into the first equation, and it will be an equation in x only. Solve it.

  • algebra -

    7x+y=-8
    i don't understand what you mean. could you possibly do an example?

  • algebra -

    We have this:

    How do you use the substitution method to solve the linear system?

    -2x - 5y = 7...Equation A

    7x - y = -8...Equation B

    The idea is to isolate y OR x (your choice) and then plug that into EITHER Equation A or B (your choice).

    I will isolate y in Equation B.

    7x - y = -8...Equation B

    -y = -7x - 8

    y = (-7x - 8)/-1

    y = 7x + 8

    Do you see that we NOW know that y is 7x + 8?

    To find x, I will plug 7x + 8 into EITHER Equation A or B.

    I will choose Equation A.

    -2x-5y = 7...Equation A

    -2x - 5(2x + 8) = 7

    -2x - 10x - 40 = 7

    -12x - 40 = 7

    -12x = 7 + 40

    -12x = 47

    x = 47/-12

    x = -47/12...Value for x.

    We just found the value of x and it is
    -47/12. See it above?

    We now plug our value for x into EITHER Equation A or B to find the value of y.

    I will choose Equation A again.

    -2x -5y = 7...Equation A

    -2(-47/12) - 5y = 7

    47/6 - 5y = 7

    -5y = -47/6 + 7

    -5y = -5/6

    y = -5/6 divided by -5

    y = 1/6

    The solution for this system of linear equations in two variables is
    x = -47/12 and y = 1/6.

    We can also write the solution this way:

    (-47/12, 1/6) = (x, y), where x represents -47/12 and y represents 1/6.

    Done!

  • algebra -

    whats the answer to -2x-5y=7
    7x+y=-8

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