plis tolong bantu jawab no 21, 22 dan 26 dengan menggunakan cara

21. 2x + z = 5
y - 2z = - 3
x + y = 1 ~ x = 1 - y
Ditanya : x + y + z = ....
Jawab :

Substitusi persamaan 1
2(1 - y) + z = 5
2 - 2y + z = 5
-2y + z = 5 - 2
-2y + z = 3

Eliminasi persamaan 4 dan 2
- 2y + z = 3 |×2
y - 2z = - 3 |×1
____________
- 4y + 2z = 6
y - 2z = - 3
___________ +
-3y = 3
y = - 1

Substitusi persamaan 3
x + y = 1
x + (-1) = 1
x = 1 + 1
x = 2
Substitusi persamaan 1
2x + z = 5
2(2) + z = 5
4 + z = 5
z = 5 - 4
z = 1
Nilai x + y + z = 2 - 1 + 1 = 2


22. 3x - 2y = 15
5x + 4y = 3
2x - 5z = - 44
Nilai x + 4y = z = ....

3x - 2y = 15 | ×2
5x + 4y = 3 | ×1
_____________
6x - 4y = 30
5x + 4y = 3
_________ +
11x = 33
x = 3
Substitusi persamaan 3
2x - 5z = - 44
2(3) - 5z = - 44
6 - 5z = - 44
-5z = - 44 - 6
-5z = - 50
z = 10
Substitusi persamaan 1
3x - 2y = 15
3(3) - 2y = 15
9 - 2y = 15
-2y = 6
y = - 3
Nilai x + 4y + z = 3 + 4(-3) + 10 = 1