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题是 求3个equation的值 x1 x2 x3 为3个变量 但是中间有一步没看懂

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题是 求3个equation的值 x1 x2 x3 为3个变量 但是中间有一步没看懂
2*x1+4*x2-3*x3=1
x1+x2+2*x3 =9
3*x1+6*x2-5*x3=0
Swapping the first and second equation,we have
x1+x2+2*x3 =9
2*x1+4*x2-3*x3=1
3*x1+6*x2-5*x3=0
Next,we subtract 2 lots of the first equation from the second and 3 lots of the first equation from the third to obtain )
x1+x2+2*x3 =9
2*x2 -7*x3 = -17 (这个从哪里来的?怎么算出来的?)
3*x2 -11*x3 = -27(这个又是从哪里来的?怎么算出来的?)
x1+x2+2*x3 =9 (1)
2*x1+4*x2-3*x3=1 (2)
3*x1+6*x2-5*x3=0 (3)
这句话意思是:
(2)-(1)×2得 2*x2 -7*x3 = -17
(3)-(1)×3得 3*x2 -11*x3 = -27