Question

若方程组x+2y=k,3x+5y=k+1的解满足x+y=5,则k的值为?

Answer 1

x=k-2y

3(k-2y)+5y=k+1

-y=-2k+1

y=2k-1

x=k-2(2k-1)

x=-3k+2

x+y=-3k+2+2k-1=5

k=-4