七年级数学解方程组8

1、二元一次方程组的解法二元一次方程组的解法七年级数学七年级数学(下下)加减法加减法(3)解方程组解方程组:3x - y = 3,4x+3y = 17.+,得得解解3,得得x = 2.即即即即32-y = 3,y = 3.所以所以x = 2,y = 3.9x-3y = 9,4x +3y = 17.13x = 26,将将x=2代入代入,得得6-y = 3,-y = 3-6.(1)消去消去y或将或将x=2代入代入,得得42+3y = 17,8+3y = 173y =17-8,3y = 9,y = 3.解方程组解方程组:3x - y =3,4x + 3y = 17. - ,得得解解4,得得y = 3.即

2、即即即3x-3 = 3,所以所以x = 2,y = 3.12x - 4y = 12,12x + 9y = 51.9y-(-4y) = 51-12,将将y = 3代入代入,得得3x = 3+3,x = 2 .(1)消去消去x3,得得13y = 39,解方程组解方程组: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(2)9x - 12y = 30,10 x+12y = 84.+ ,得得56+6y = 42,30+6y = 42,6y = 42-30,6y = 12,消去

3、消去y解方程组解方程组:3x - 4y = 10,5x+6y = 42. 解解y = 2.即即即即所以所以x= 6,y= 2.38y = 76,把把y= 2代入代入,得得x= 6. 5,得得 3,得得(2)15x - 20y = 50,15x+18y = 126.- ,得得3x - 42 = 10,3x-8 = 10,3x = 10+8,3x= 18,消去消去x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= 4代入代入,得得y = 3. 3,得得 2,得得(3)9x - 6y = 18,4x+6

4、y = 34. + ,得得2 4 + 3 y = 17,8 + 3y= 17,3y = 17-8,3y = 9,消去消去y解方程组解方程组:3x - 2y = 6,2x+3y = 17. 解解y = 3.即即即即所以所以x = 4,y = 3.13y = 39,把把y=3代入代入,得得x = 4. 2,得得 3,得得(3)6x - 4y = 12,6x+9y = 51. - ,得得2x+3 3 = 17,2x + 9= 17,2x = 17-9,2x = 8,消去消去x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.

5、-11x = 55,把把x=-5代入代入,得得y = -6. 5,得得 3,得得(4)10 x - 15y = 40,15y-21x = 15.+,得得5y-7(-5) = 5,5y+35 = 5,5y = 5-35,5y = -30,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(4)10 x - 15y = 40,-21x+15y = 15.+,得得5y-7(-5) = 5,5y+35 = 5,5y = 5-35,5y =

6、 -30,消去消去y原方程组可化为原方程组可化为2x - 3y = 8,-7x+5y = 5.解方程组解方程组:2x - 3y = 8,5y-7x = 5.解解y = -6.即即即即所以所以x = -5,y = -6.-11y = 66,把把y= - 6代入代入,得得x = -5. 7,得得 2,得得(4)14x - 21y = 56,-14x+10y = 10.+,得得2x - 3(-6) = 8,2x+18 = 8,2x = 8-18,2x = -10,消去消去x原方程组可化为原方程组可化为2x - 3y = 8,-7x+5y = 5.解方程组解方程组:2x-7y = 10,3x-8y-

7、10 =0.解解2x-7 ( ) = 10,5y = -10,即即所以所以x = -2,y = -2.2x+14=10,把把y=-2代入代入,得得y = -2.(5)3x-8y = 10. 2,得得3,得得6x-16y = 20,6x-21y = 30.- ,得得-16y-(-21y) = 20-30,-22x =10-14,2x = -4,x = -2.消去消去x原方程组可化为原方程组可化为2x-7y = 10,解二元一次方程组的基本思想是解二元一次方程组的基本思想是_,化化_为为_;基本方法有基本方法有_、 _.消元消元代入法代入法加减法加减法二元二元一元一元做一做做一做:用加减法解方程组用加减法解方程组.1123, 642) 1 (yxyx. 153, 732)2(yxyx.41,27yx. 1, 2yx. 010073, 0203)3(yxyx.10,10yx(附加题附加题)._,73. 124277yxbabaxyyx则是同类项和如果.27,427xy