ax+y+2z=5 ....(1) bx-y+3z=3 ....(2) cx-y+z=-1 .....(3) jika a+b=7 dan a+c=5 nilai 12x+8z=.........??? a.8 b.10 c.12 d.16 e.18

(1) dan (2)
ax+y+2z=5
bx-y+3z=3
_________ +
ax+bx+5z=8
x(a+b)+5z=8
a+b = 8-5z/x = 7
             8-5z = 7x
                  8 = 7x+5z ...(4)
(1) dan (3)
ax+y+2z=5
   cx-y+z=-1
_________ +
ax+cx+3z=4
     x(a+c) = 4-3z
         a+c = 4-3z/x = 5
                      4-3z = 5x
                           4 = 5x+3z .....(5)
(4) dan (5)
7x+5z=8
5x+3z=4
_______ +
12x+8z=12

ax+y+2z=5........(1)
bx-y+3z=3.........(2)
cx-y+z=-1..........(3)
a+b=7 => a=7-b
a+c=5 => c=5-a
Eliminasi (1) & (2)
ax+y+2z=5
bx-y+3z=3
____________+
ax+bx+5z=8
(7-b)x+bx+5z=8
7x+5z=8..............(4)
Eliminasi (3) & (1)
cx-y+z=-1
ax+y+2z=5
_____________+
cx+ax+3z=4
(5-a)x+ax+3z=4
5x+3z=4.............(5)
Eliminasi (4) dan (5)
7x+5z=8 |×3| 21x+15z=24
5x+3z=4 |×5| 25x+15z=20
______________-
-4x = 4
x = -1
Subtitusi x ke pers (4)
7x+5z=8
7(-1)+5z=8
-7+5z=8
5z = 15
z = 3
Nilai 12x+8z = 12(-1)+8(3)
= -12+24
= 12 (C)