{x+2y=k ①{3x+5y=k-1 ②由②得3x+5y+1=k∴x+2y=3x+5y+1 ③∵x与y的差是5∴x=y-5或y=x-5若x=y-5,代入③得：y-5+2y=3y-15+5y+1∴-5y=-9∴y=1.8∴x=y-5=1.8-5=-3.2把y=1.8,x=-3.2代入①得：-3.2+3.6=k∴k=0.4若y=x-5,...