※ 引述《TOMOHISA (YAMASHITA)》之銘言：
: 解四元一次聯立方程組，其中a~h為常數：
: X = a + b + X + X
: 1 2 3
: X = c + d + X + X
: 2 1 4
: X = e + f + X + X
: 3 1 4
: X = g + h + X + X
: 4 2 3
: 得X =____________。
: 1
X1 = a + b + X2 + X3 ....(1)
X2 = c + d + X1 + X4 ....(2)
X3 = e + f + X1 + X4 ....(3)
+) X4 = g + h + X2 + X3 ....(4)
__________________________
(X1+X2+X3+X4) = 2(X1+X2+X3+X4) + a + b + c + d + e + f + g + h
=> (X1+X2+X3+X4) = -(a + b + c + d + e + f + g + h)
(1)+(2) => X3 + X4 = -(a + b + c + d) => X1 + X2 = -(e + f + g + h) ...(5)
(1)-(4)-(2) => 2*X1 - X2 = a + b - c - d - g - h ...(6)
(5)+(6) => 3*X1 = a + b - c - d - e - f - 2g - 2h
=> X1 = (a + b - c - d - e - f - 2g - 2h)/3
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