[[[20點]]急!!!聯立方程([[[[[[[[[[加減消元法]]]]]]]]]]]])

6.
3x+y = 5 ...... (1)
2x+5y = 4 ...... (2)

(1)*5： 15x+5y = 25 ......(3)
(2): 2x+5y = 4 ...... (2)
(3)-(2): 13x = 21
x = 21/13

(1)*2: 6x+2y = 10 ...... (4)
(2)*3: 6x+15y = 12 ...... (5)
(5)-(4): 13y = 2
y = 2/13

答: x=21/13, y=2/13

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7.
6x+y = 4 ...... (1)
7x+2y = 3 ...... (2)

(1)*2: 12x+2y = 8 ...... (3)
(3): 7x+2y = 3 ...... (2)
(3)-(2): 5x = 5
x = 1

(1)*7: 42x+7y = 28 ...... (4)
(2)*6: 42x + 12y = 18 ...... (5)
(5)-(4): 5y = -10
y = -2

答：x=1, y = -2

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8.
5x+2y = 1 ...... (1)
2y-x = 7 ...... (2)

(1): 5x+2y = 1 ...... (1)
(2): -x+2y = 7 ...... (2)
(1)-(2): 6x = -6
x = -1

(1): (1): 5x+2y = 1 ...... (1)
(2)*5: -5x+10y = 35 ...... (3)
(1)+(3): 12y = 36
y = 3

答：x=-1, y=3

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9.
3y-4x = 7 ...... (1)
x+5y = 4 ...... (2)

(1)*5: -20x+15y = 35 ...... (3)
(2)*3: 3x+15y = 12 ...... (4)
(3)-(4): -23x = 23
x = -1

(2)*4: 4x+20y = 16 ...... (5)
(1): -4x+3y = 7 ...... (1)
(5)+(1): 23y = 23
y = 1

答：x=-1, y=1
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