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1、第7章 二元一次方程組,七年級(jí)數(shù)學(xué)(下),加減法(2),解方程組:,,7x-2y=3,,9x+2y= -19.,,,解,y= -5.,即,即,9(-1) +2y = -19,,x= -1.,所以,,x=-1,,y=,+ ,得,將x= -1代入,得,(1),7x+9x=3+(-19),,16x= -16,,2y = -19+9,-5.,2y = -10,,消去y,解方程組:,,5x+2y =1,,3x+2y = 3.,,,解,y= 3.,即,即,3(-1) +2y = 3,,x = -1.,所以,,x = -1,,y =,- ,得,將x= -1代入,得,(2),5x-3x = 1-3,,2x =

2、 -2,,2y = 3+3,3.,2y = 6,,消去y,解方程組:,,3x-5y = 6,,x+4y = -15.,,, - ,得,解,3,得,,17y = -51,,y = -3.,即,即,x+4 ( )= -15,,-3,x = -3,所以,,x = -3,,y = -3.,3x+12y = -45,3x - 5y = 6,,12y-(-5y) = -45-6,,將y=-3代入,得,x-12= -15,,(3),消去x,解方程組:,,x-3y = -20,,3x+7y =100,,, - ,得,解,3,得,,16y = 160,,y =10.,即,即,x-3 = -20,10,x =

3、10.,所以,,x = 10,,y = 10.,3x-9y = -60,,3x +7y = 100.,,7y-(-9y) = 100-(-60),,將y=10代入,得,x-30 = -20,,(4),消去x,解方程組:,,4x-2y =14,,5x+y = 7.,,,+,得,解,2,得,,14x = 28,,x = 2.,即,即,52+y = 7,,y = -3.,所以,,x = 2,,y = -3.,10 x+2y = 14,,4x - 2y = 14.,,10 x+4x = 14+14,,將x=2代入,得,10+y = 7,,(5),消去y,做一做:用加減法解方程組,解方程組:,,3x -

4、 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,(6),9x - 12y = 30,,10 x+12y = 84.,,,+ ,得,56+6y = 42,,30+6y = 42,,6y = 42-30,,6y = 12,,消去y,解方程組:,,3x - 2y = 6,,2x+3y = 17.,,,解,x = 4.,即,即,所以,,x = 4,,y = 3.,13x = 52,,把x=3代入,得,y = 3., 3,得, 2,得,(7),9x - 6y = 18,

5、,4x+6y = 34.,,,+ ,得,24+3y = 17,,8+3y = 17,,3y = 17-8,,3y = 9,,消去y,解方程組:,,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,(8),10 x - 15y =40,,15y-21x = 15.,,,+,得,5y-7(-5 ) = 5,,5y+35 = 5,,5y = 5-35,,5y = -30,,消去y,解方程組:,,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.,(9),3x-8y = 10.,由,得,,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,做一做:用加減法解方程組,(附加題),