1、8.2 消元-二元一次方程组的解法（二）,复习,1、标 和,2、变形和标,3、代入并求解,4、再代,5、写解,解：,原方程组可化为：,由得：,x = 2 3y,把代入得：,-2（2 3y）+ 6y = 1,-4 + 6y + 6y = 1,6y + 6y = 1 + 4,12y = 5,y = 5/12,把y = 5/12 代入，得,x = 2 3y,= 2 - 35/12,x = 3/4,x = 3/4,y = 5/12,解：,原方程组可化为：,3x 2y = 6,x y = 2,由得：,把代入得：,x = 2 + y,3（2 + y）- 2y = 6,6 + 3y 2y = 6,y = 0
2、,把y = 0 代入，得：,x = 2 + y,x = 2,x = 2,y = 0,解方程组：,3.练 习 题,若方程5x 2m+n+4y 3m-2n = 9是关于x、y的二元一次方程，求m 、n 的值.,解：,根据已知条件得：,2m + n = 1,3m 2n = 1,由得：,把代入得：,n = 1 2m,3m 2（1 2m）= 1,3m 2 + 4m = 1,7m = 3,m = 3/7,把m = 3/7 代入，得：,n = 1 2m,4.思 考 题,已知 是关于 x、y 的方程组 的解， 求 a 、 b 的值.,x =-1， y = 2，,-2 + 2a = 3b,-a 2b = 1,5
3、.思 考 题,由得：,a = -2b - 1,把代入得：,-2 + 2（-2b 1）= 3b,-2 4b 2 = 3b,-4b 3b = + 2 + 2,-7b = 4,b = -4/7,把b = -4/7 代入，得：,a = 1/7,a = 1/7,b = -4/7,a = -2b - 1,= -2（-4/7）-1,6.思 考 题,解：,由得：,y = 2x - 3,把代入得：,3x + 2（2x 3）= 8,3x + 4x 6 = 8,3x + 4x = 8 + 6,7x = 14,x = 2,把x = 2 代入，得：,y = 2x - 3,= 22 - 3,=1,解得：,a = 1,b = -1,7.思 考 题,如果y + 3x - 2+5x + 2y -2= 0，求 x 、y 的值.,解：,根据已知条件，得：,由得：,y = 2 3x,把代入得：,5x + 2（2 3x）- 2 = 0,5x + 4 6x 2 = 0,5x 6x = 2 - 4,-x = -2,x = 2,把x = 2 代入，得：,y = 2 3x,= 2 - 32,= -4,答：x 的值是2，y 的值 是-4.,
