选用适当方法解二元一次方程组

1、二元一次方程组的解法,七年级数学(下),加减法(3),解方程组:,3x - y = 3,4x+3y = 17.,+,得,解,3,得,x = 2.,即,即,32-y = 3,y = 3.,所以,x = 2,y = 3.,9x-3y = 9,4x +3y = 17.,13x = 26,将x=2代入,得,6-y = 3,-y = 3-6.,(1),消去y,或将x=2代入,得,42+3y = 17,8+3y = 17,3y =17-8,3y = 9,y = 3.,解方程组:,3x - y =3,4x + 3y = 17., - ,得,解,4,得,y = 3.,即,即,3x-3 = 3,所以,x = 2

2、,y = 3.,12x - 4y = 12,12x + 9y = 51.,9y-(-4y) = 51-12,将y = 3代入,得,3x = 3+3,x = 2 .,(1),消去x,3,得,13y = 39,解方程组:,3x - 4y = 10,5x+6y = 42.,解,x = 6.,即,即,所以,x= 6,y= 2.,19x = 114,把x=6代入,得,y= 2., 3,得, 2,得,(2),9x - 12y = 30,10 x+12y = 84.,+ ,得,56+6y = 42,30+6y = 42,6y = 42-30,6y = 12,消去y,解方程组:,3x - 4y = 10,5x

3、+6y = 42.,解,y = 2.,即,即,所以,x= 6,y= 2.,38y = 76,把y= 2代入,得,x= 6., 5,得, 3,得,(2),15x - 20y = 50,15x+18y = 126.,- ,得,3x - 42 = 10,3x-8 = 10,3x = 10+8,3x= 18,消去x,解方程组:,3x - 2y = 6,2x+3y = 17.,解,x= 4.,即,即,所以,x = 4,y = 3.,13x = 52,把x= 4代入,得,y = 3., 3,得, 2,得,(3),9x - 6y = 18,4x+6y = 34., + ,得,2 4 + 3 y = 17,8

4、 + 3y= 17,3y = 17-8,3y = 9,消去y,解方程组:,3x - 2y = 6,2x+3y = 17.,解,y = 3.,即,即,所以,x = 4,y = 3.,13y = 39,把y=3代入,得,x = 4., 2,得, 3,得,(3),6x - 4y = 12,6x+9y = 51., - ,得,2x+3 3 = 17,2x + 9= 17,2x = 17-9,2x = 8,消去x,解方程组:,2x - 3y = 8,5y-7x = 5.,解,x = -5.,即,即,所以,x = -5,y = -6.,-11x = 55,把x=-5代入,得,y = -6., 5,得, 3

5、,得,(4),10 x - 15y = 40,15y-21x = 15.,+,得,5y-7(-5) = 5,5y+35 = 5,5y = 5-35,5y = -30,消去y,解方程组:,2x - 3y = 8,5y -7x = 5.,解,x = -5.,即,即,所以,x = -5,y = -6.,-11x = 55,把x=-5代入,得,y = -6., 5,得, 3,得,(4),10 x - 15y = 40,-21x+15y = 15.,+,得,5y-7(-5) = 5,5y+35 = 5,5y = 5-35,5y = -30,消去y,原方程组可化为,2x - 3y = 8,-7x+5y =

6、 5.,解方程组:,2x - 3y = 8,5y-7x = 5.,解,y = -6.,即,即,所以,x = -5,y = -6.,-11y = 66,把y= - 6代入,得,x = -5., 7,得, 2,得,(4),14x - 21y = 56,-14x+10y = 10.,+,得,2x - 3(-6) = 8,2x+18 = 8,2x = 8-18,2x = -10,消去x,原方程组可化为,2x - 3y = 8,-7x+5y = 5.,解方程组:,2x-7y = 10,3x-8y- 10 =0.,解,2x-7 ( ) = 10,5y = -10,即,所以,x = -2,y = -2.,2x+14=10,把y=-2代入,得,y = -2.,(5),3x-8y = 10., 2,得,3,得,6x-16y = 20,6x-21y = 30.,- ,得,-16y-(-21y) = 20-30,-2,2x =10-14,2x = -4,x = -2.,消去x,原方