華師大版7.2二元一次方程組的解法(加減法2)課件

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1、七年級(jí)數(shù)學(xué)(下)加減法(2)Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.解方程組:7x-2y=3,9x+2y=-19.②①解y=-5.即即9×(-1)+2y=-19,x=-1.所以x=-1,y=①+②,得將x=-1代入②,得(1)7x+9x=3+(-19),16x=-16,2y=-19+9-5.2y=-10,消去yEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5Clie

2、ntProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.解方程組:5x+2y=1,3x+2y=3.②①解y=3.即即3×(-1)+2y=3,x=-1.所以x=-1,y=①-②,得將x=-1代入②,得(2)5x-3x=1-3,2x=-2,2y=3+33.2y=6,消去yEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.解方程組:3x-5y=6,x+4y=-15.②①③-①,得解②×3,得

3、③17y=-51,y=-3.即即x+4×()=-15,-3x=-3所以x=-3,y=-3.3x+12y=-453x-5y=6①12y-(-5y)=-45-6,將y=-3代入②,得x-12=-15,(3)消去xEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.解方程組:x-3y=-20,3x+7y=100.②①②-③,得解①×3,得③16y=160,y=10.即即x-3×=-2010x=10.所以x=10,y=10.3x-9y

4、=-60,3x+7y=100.②7y-(-9y)=100-(-60),將y=10代入①,得x-30=-20,(4)消去xEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.解方程組:4x-2y=14,5x+y=7.②①③+①,得解②×2,得③14x=28,x=2.即即5×2+y=7,y=-3.所以x=2,y=-3.10x+2y=14,4x-2y=14.①10x+4x=14+14,將x=2代入②,得10+y=7,(5)消去yEv

5、aluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.做一做:用加減法解方程組Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.解方程組:3x-4y=10,5x+6y=42.②①解x=6.即即所以x=6,y=2.19x=114,把x=6代入②,得y=2.①×3,得②×2,得(6)9x

6、-12y=30,10x+12y=84.③④③+④,得5×6+6y=42,30+6y=42,6y=42-30,6y=12,消去yEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.解方程組:3x-2y=6,2x+3y=17.②①解x=4.即即所以x=4,y=3.13x=52,把x=3代入②,得y=3.①×3,得②×2,得(7)9x-6y=18,4x+6y=34.③④③+④,得2×4+3y=17,8+3y=17,3y=17-8,3

7、y=9,消去yEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile5.2.0.0.Copyright2004-2011AsposePtyLtd.解方程組:2x-3y=8,5y-7x=5.②①解x=-5.即即所以x=-5,y=-6.-11x=55,把x=-5代入②,得y=-6.①×5,得②×3,得(8)10x-15y=40,15y-21x=15.③④③+④,得5y-7×(-5)=5,5y+35=5,5y=5-35,5y=-30,消去yEvalua