Question

:已知方程组x+y=2k,y+z=4k,z+x=8k的解可使代数式2x-y-3z的值为-20,求k的值.

Answer 1

x+y=2k (1)

y+z=4k (2)

z+x=8k (3)

(1)+(2)+(3)

2x+2y+2z=14k

x+y+z=7k (4)

(4)-(1)

z=5k

(4)-(2)

x=3k

(4)-(3)

y=-k

2x-y-3z=6k+k-15k=-8k=-20

k=5/2