二元一次方程组的解法七年级数学 ( 下 )加减法 (2) 解方程组 :7x-2y=3,9x+2y= -19. ②①解y= -5.即即9×(-1) +2y = -19,x= -1.所以x=-1,y= ①+ ②, 得将 x= -1 代入② , 得(1)7x+9x=3+(-19),16x= -16,2y = -19+9-5.2y = -10,消去y 解方程组 :5x+2y =1,3x+2y = 3.②①解y= 3.即即3×(-1) +2y = 3,x = -1.所以x = -1,y = ①- ②, 得将 x= -1 代入② , 得(2)5x-3x = 1-3,2x = -2,2y = 3+33.2y = 6,消去y 解方程组 :3x-5y = 6,x+4y = -15.②① ③ - ,① 得解②×3, 得③17y = -51,y = -3.即即x+4× ( )= -15,-3x = -3所以x = -3,y = -3.3x+12y = -453x - 5y = 6①12y-(-5y) = -45-6,将 y=-3 代入② , 得x-12= -15,(3)消去x 解方程组 :x-3y = -20,3x+7y = 100.②① ② - ,③ 得解①×3, 得③16y = 160,y =10.即即x-3× = -2010x =10.所以x = 10,y = 10.3x-9y = -60,3x +7y = 100.②7y-(-9y) = 100-(-60),将 y=10 代入① , 得x-30 = -20,(4)消去x 解方程组 :4x-2y =14,5x+y = 7.②①③+,① 得解②×2, 得③14x = 28,x = 2.即即5×2+y = 7,y = -3.所以x = 2,y = -3.10x+2y = 14,4x - 2y = 14.①10x+4x = 14+14,将 x=2 代入② , 得10+y = 7,(5)消去y 做一做 : 用加减法解方程组18223)1(yxyx.634,02)2(yxyx.2,8yx.6,3yx 解方程组 :3x - 4y = 10,5x+6y = 42. ②①解x = 6.即即所以x = 6,y = 2.19x = 114,把 x=6 代入② , 得y = 2.① ×3, 得 ② ×2, 得(6)9x - 12y = 30,10x+12y = 84.③④③+ ,④ 得5×6+6y = 42,30+6y = 42,6y = 42-30,6y = 12,消去y 解方程组 :3x - 2y = 6,2x+3y = 17. ②①解x = 4.即即所以x = 4,y = 3.13x = 52,把 x=3 代入② , 得y = 3.① ×3, 得 ② ×2, 得(7)9x - 6y = 18,4x+6y = 34.③④③+ ,④ 得2×4+3y = 17,8+3y = 17,3y = 17-8,3y = 9,消去y 解方程组 :2x - 3y =8,5y-7x = 5. ②①解x = -5.即即所以x = -5,y = -6.-11x = 55,把 x=-5 代入② , 得y = -6.① ×5, 得 ② ×3, 得(8)10x - 15y =40,15y-21x = 15.③④③+,④ 得5y-7×(-5 ) = 5,5y+35 = 5,5y = 5-35,5y = -30,消去y 解方程组 :2x-7y =10,3x-8y- 10 = 0. ②①解2x-7( ) = 10,5y = -10,即所以x = -2,y = -2.2x+14 = 10,把 y=-2 代入① , 得y = -2.(9)3x-8y = 10.由② , 得③③×2, 得①×3, 得6x-16y = 20,6x-21y = 30.④⑤④- ,⑤ 得 -16y-(-21y) = 20-30,-2 2x = 10-14,2x = -4,x = -2.消去x 做一做 : 用加减法解方程组.1123,642)1(yxyx.153,732)2(yxyx.1,5yx.1,2yx.010073,0203)3(yxyx.10,10yx( 附加题 ) 作业
