Homework Help: math(algebra 1)

Posted by please help!!! on Tuesday, January 18, 2011 at 10:53pm.

i dont get these please help me:
use the subtitution method to solve the linear system
1)3x+y=0 x-y=4
2)y=-3x-1 x-3y=3
3)3x-y=9 2x+y=6
4)x-2y=0 3x+y=0
Thank you!!:)

math(algebra 1) - helper, Tuesday, January 18, 2011 at 11:17pm
1)3x + y = 0, x - y = 4
Pick and equation, and solve for either x or y

x - y = 4
x = 4 + y

Substitute, x = 4 + y, for x, in the other equation and solve for y
x = 4 + y
3x + y = 0
3(4 + y) + y = 0
12 + 3y + y = 0
12 + 4y = 0
4y = -12
y = -3

substitute y = -3 in either equation to find x
x - y = 4
x - (-3) = 4
x + 3 = 4
x = 1

Now you try the others
math(algebra 1) - please help!!, Tuesday, January 18, 2011 at 11:47pm
so are these correct?
2)y=-3x-1 x-3y=3
x=0 y=-1???
3)3x-y=9 2x+y=6
dont know help??
4)x-2y=0 3x+y=0
please help!!! i dont know how you changed x - y = 4 to x = 4 + y tell me please
Thanks!!
math(algebra 1) - helper, Wednesday, January 19, 2011 at 12:57pm
2) you are correct

x - y = 4
add + y to both sides
x - y + y = 4 + y
x = 4 + y

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