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)代入法代入法(3)Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.解方程组解方程组:x-y=1,2x+y=5.x=2,y=1.(1)(2)x=5,y=-2. x+y=3,x-y=7.练习练习(3)(4)x=y+1.4x+3y=17.2x= y-1,3x-y=3,y=-3.y=3.x=2,x=-2,Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.例例解方程组解方程组2x-7y=8,3x-8y-10=0.解解由由,得得x=4+ 3.5 y.将将代入代入,得得 3 ( ) -8y-10=0,4+ 3.5 y- 8y - 10=0,2.5y=-2,即即y=-0.8将将y=-0.8代入代入,得得x=4+ 3.5(-0.8) 即即x=1.2所以所以x=1.2y=-0.8代入法代入法12 + 10.5y10.5y- 8y= 10-12,2x=8+7y,Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.练习练习课本第课本第30页第页第1题题1.把下列各方程变形为用一个未知数表示另一个未知数的形式把下列各方程变形为用一个未知数表示另一个未知数的形式:(1) 4x-y= -1;(2) 5x-10y+15=0.解解:(1) 4x-y= -1,-y= -1-4x,y=1+4x.或或4x-y= -1,4x= -1+y,x=4-1+y(2) 5x-10y+15=05x=10y-15x=2y-3或或 -10y= -5x-15y=-10-5x-15(用用x表示表示y)(用用x表示表示y)(用用y表示表示x)(用用y表示表示x)Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.解方程组解方程组:x-3y+20=0,3x+7y-100=0.x=10,y=10.作业作业(3)课本第课本第34页习题页习题7.2第第1题题(3)(4)(4)x=-2,y=5.2y-8= -x,4x+3y=7.附加题附加题:解方程组解方程组Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.作业Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.