Diketahui x1 dan y1 memenuhi persamaan 2x – 3y = 7  dan 3x – 4y = 9 Nilai x1 + y1 = ???????

2x – 3y = 7  | 3|  6x – 9y = 21
3x – 4y = 9  | 2|  6x – 8y = 18  -
                                     y = – 3


2x – 3y = 7     
2x – 3.(-3) = 7     
2x + 9 = 7            
2x = – 2              
x = – 1     
Jadi  x1 + y1 = ( – 1 ) + ( – 3 ) = – 4

2x - 3y = 7 x 3
3x - 4y = 9 x 2

6x - 9y = 21
6x - 8y = 18
        y = -3

2x - 3y = 7
2x - 3 (-3) = 7
2x + 9 = 7
2x      = 7 - 9
2x = -2
x = -1

jadi x1+y1 = (-3) + (-1) = -4