华师大版72二元一次方程组的解法-ppt课件

1、七年级数学七年级数学( (下下) )加减法加减法(2)解方程组解方程组:7x-2y=3,9x+2y= -19. 解解y= -5.即即即即9(-1) +2y = -19,x= -1.所以所以x=-1,y= + ,得得将将x= -1代入代入,得得(1)7x+9x=3+(-19),16x= -16,2y = -19+9-5.2y = -10,消去消去y解方程组解方程组:5x+2y =1,3x+2y = 3.解解y= 3.即即即即3(-1) +2y = 3,x = -1.所以所以x = -1,y = - ,得得将将x= -1代入代入,得得(2)5x-3x = 1-3,2x = -2,2y = 3+33

2、.2y = 6,消去消去y解方程组解方程组:3x-5y = 6,x+4y = -15. - ,得得解解3,得得17y = -51,y = -3.即即即即x+4 ( )= -15,-3x = -3所以所以x = -3,y = -3.3x+12y = -453x - 5y = 612y-(-5y) = -45-6,将将y=-3代入代入,得得x-12= -15,(3)消去消去x解方程组解方程组:x-3y = -20,3x+7y = 100. - ,得得解解3,得得16y = 160,y =10.即即即即x-3 = -2010 x =10.所以所以x = 10,y = 10.3x-9y = -60,3

3、x +7y = 100.7y-(-9y) = 100-(-60),将将y=10代入代入,得得x-30 = -20,(4)消去消去x解方程组解方程组:4x-2y =14,5x+y = 7.+,得得解解2,得得14x = 28,x = 2.即即即即52+y = 7,y = -3.所以所以x = 2,y = -3.10 x+2y = 14,4x - 2y = 14.10 x+4x = 14+14,将将x=2代入代入,得得10+y = 7,(5)消去消去y做一做做一做:用加减法解方程组用加减法解方程组18223) 1 (yxyx. 634, 02)2(yxyx. 2, 8yx. 6, 3yx解方程组解

4、方程组:3x - 4y = 10,5x+6y = 42. 解解x = 6.即即即即所以所以x = 6,y = 2.19x = 114,把把x=6代入代入,得得y = 2. 3,得得 2,得得(6)9x - 12y = 30,10 x+12y = 84.+ ,得得56+6y = 42,30+6y = 42,6y = 42-30,6y = 12,消去消去y解方程组解方程组:3x - 2y = 6,2x+3y = 17. 解解x = 4.即即即即所以所以x = 4,y = 3.13x = 52,把把x=3代入代入,得得y = 3. 3,得得 2,得得(7)9x - 6y = 18,4x+6y = 34.+ ,得得24+3y = 17,8+3y = 17,3y = 17-8,3y = 9,消去消去y解方程组解方程组:2x - 3y =8,5y-7x = 5. 解解x = -5.即即即即所以所以x = -5,y = -6.-11x = 55,把把x=-5代入代入,得得y = -6. 5,得得 3,得得(8)10 x - 15