introductory algebra

I had these problems on my test and for some reason got them wrong. Here are the problems:

Solve:
x-y=9
x+y=1

The test states that the answer is (5,-4)

I've checked ,y work and used substitution to solve this and still get x=4, y=-5.



The second problem is:
Solve the system of equations by graphing.
a=1/2b+5
a-4b=-2

The answer is (6,2)

Again i got the reverse of this; (2,6).

I have been at these for several hours and want to pull my hair out. Are my answers really wrong or is it the test? HELP!


Thanks

asked by jose
  1. x-y=9
    x+y=1

    The test states that the answer is (5,-4)

    I've checked ,y work and used substitution to solve this and still get x=4, y=-5.
    ------------------------
    Check your answer
    4 - -5 = 9 yes
    4 + -5 = -1, not +1 wrong
    Now adding the two equations
    2 x = 10
    x = 5
    then
    5+y = 1
    y = -4

    posted by Damon
  2. a=1/2b+5
    a-4b=-2

    substitute (1/2) b + 5 for a

    (1/2) b + 5 - 4 b = -2

    b + 10 - 8b = -4

    -7 b = -14
    b = 2
    then
    a = 1 + 5 = 6

    posted by Damon

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