解方程组(加减法)

课件20张PPT。二元一次方程组的解法------------加减法黔江区舟白中学--粟永镇E-Mail:416352289@qq.com解方程组（1）3x+7y=9,4x-7y= 5.②①解解得即所以x=2,y=y=由①+ ②,得7x=14,x=2.将x=2代入①,得3×2+7y=9,消去y解方程组:x+y=7,3x+y= 17.②①解x=5.即解得所以x=5,y=2.2x=10,把x=5代入①,得y=2.由②- ①,得5+y=7,(2)解方程组:7x-2y=3,9x+2y= -19.②①解y= -5.解得解得9×(-1) +2y = -19,x= -1.所以x= -1,y= -5 由①+ ②,得将x= -1代入②,得(3)7x+9x=3+(-19),消去y解方程组:5x+2y =1,3x+2y = 3.②①解y= 3.解得即3×(-1) +2y = 3,x = -1.所以x = -1,y = 3 由①- ②,得将x= -1代入②,得(4)5x-3x = 1-3,消去y课本第32页第1、2、3、4题练习解方程组:3x-5y = 6,x+4y = -15.②①由③ - ①,得解由②×3,得③y = -3.解得即x+4× ( )= -15,-3x = -3所以x = -3,y = -3.3x+12y = -453x - 5y = 6①12y-(-5y) = -45-6,将y=-3代入②,得(5)消去x解方程组:x-3y = -20,3x+7y = 100.②①由② - ③,得解由①×3,得③y =10.解得即x-3× = -2010x =10.所以x = 10,y = 10.3x-9y = -60,3x +7y = 100.②7y-(-9y) = 100-(-60),将y=10代入①,得(6)消去x解方程组:4x-2y =14,5x+y = 7.②①由③+①,得解由②×2,得③x = 2.解得即5×2+y = 7,y = -3.所以x = 2,y = -3.10x+2y = 14,4x - 2y = 14.①10x+4x = 14+14,将x=2代入②,得(7)消去y做一做:用加减法解方程组解方程组:3x - 4y = 10,5x+6y = 42.②①解x = 6.解得即所以x = 6,y = 2.19x = 114,把x=6代入②,得y = 2.由① ×3,② ×2,得(8)9x - 12y = 30,10x+12y = 84.③④由③+ ④,得5×6+6y = 42,消去y解方程组:3x - 2y = 6,2x+3y = 17.②①解x = 4.解得即所以x = 4,y = 3.13x = 52,把x=3代入②,得y = 3.由① ×3,② ×2,得(9)9x - 6y = 18,4x+6y = 34.③④由③+ ④,得2×4+3y = 17,消去y解方程组:2x - 3y =8,5y-7x = 5.②①解x = -5.解得即所以x = -5,y = -6.-11x = 55,把x=-5代入②,得y = -6.由① ×5,② ×3,得(10)10x - 15y =40,15y-21x = 15.③④由③+④,得5y-7×(-5 ) = 5,消去y解方程组:2x-7y =10,3x-8y- 10 = 0.②①解2x-7( ) = 10,即所以x = -2,y = -2.把y=-2代入①,得y = -2.(11)3x-8y = 10.由②,得③由③×2,①×3,得6x-16y = 20,6x-21y = 30.④⑤由④- ⑤,得-16y-(-21y) = 20-30,-2 x = -2.消去x25x+23y=2 ①23x+25y=-2 ②解方程组：解：由①+ ②得由①- ②得即x+y=0 ③x-y=2 ④自己求解x+y=0x-y=2解二元一次方程组的基本思想是
________,化_____为_________;
基本方法有________ 、 ________.消元代入消元法加减消元法二元一元巩固练习2、若（x-2y+1)2+(x+2y-3)2=0,
则 x=___, y= ___。2-311X-2y+1=0x+2y-3=0X=1y=1解得做一做:用加减法解方程组作业数学书P34页
解下列方程组1、2、3、4同步练习册P26页
第二课时------加减消元法