Question

What is the solution to the linear system of equations?
x + 4y = –8
x – y = 7
http://s1077.photobucket.com/albums/w474/TiffanyxChambers/?action=view¤t=boringgg.gif <--- image
Possible answers:
A. (4, –3)
B. (4, 3)
C.(7, 0)
D.The system has no solution.

Answer 1

We can solve this simultaneously
x + 4y = -8 ------ eqn (1)
x - y = 7 ------ eqn (2)
Make x the subject of eqn (2)
x = 7 + y ------ eqn (3)
Substitute eqn (3) into anywhere you see x in eqn (1)
(7 + y) + 4y = -8
Group like terms on both sides
y + 4y = -8 - 7
5y = -15
Divide both sides by 3 to allow y to stand alone
y = -3

Put the answer for y into eqn (2) and solve for x
x - (-3) = 7
But - - = + so
x + 3 = 7
Make x the subject
x = 7 - 3
x = 4

Hence x = 4, y = -3 or (4, -3)

Answer 2

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