加减法解二元一次方程组

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,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+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,24+3y=17,8+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+33=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,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=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,原方程组可化为,2x-7y=1