X+Y=7 ①3x+5y=17 ②①×3得3X+3Y=21 ③②-③得2y=-4y=-2把y=-2代入①得x=9所以原方程组的解的x=9y=-2X+Y=7 ①3x+5y=17 ②由①得y=7-x ③把③代入②得3x+35-5x=17-2x=-18x=9把x=9代入③得y=-2所以原方程组的解的x=9y...