4x+4z=7,2x+3y+z=9,5x-9y+7z=8

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4x+4z=7,2x+3y+z=9,5x-9y+7z=84x+4z=7,2x+3y+z=9,5x-9y+7z=84x+4z=7,2x+3y+z=9,5x-9y+7z=84x+4z=7(1)2x+3y+

4x+4z=7,2x+3y+z=9,5x-9y+7z=8
4x+4z=7,2x+3y+z=9,5x-9y+7z=8

4x+4z=7,2x+3y+z=9,5x-9y+7z=8
4x+4z=7 (1)
2x+3y+z=9 (2)
5x-9y+7z=8 (3)
由(1)得到x+z=7/4 (4)
(2)*2+(3)得到9x-3y+9z=26 (5)
(4)*9 代入(5)得到 63/4-3y=26
解得y=-41/12
将y代入(2)得到2x+z=77/4
x+z=7/4
所以x=70/4=35/2
z=-63/4
所以x=35/2,y=-41/12,z=-63/4

第二个式子乘以3加上第三个式子得11x+10z=35,用所得的式子和第一个式子的算出x和z,然后把x,z代入第二或第三个方程,可得到y,最后结果为:x=35/2,y=-41/12,z=-63/4

这还不容易吗
好好看看书吧

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