2(x+y)-(x-3y)=13 4（x+y）+3（x-3y)=1解方程组 2(x+y)-(x-3y)=13 4（x+y）+3（x-3y)=1

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2(x+y)-(x-3y)=13 4（x+y）+3（x-3y)=1解方程组 2(x+y)-(x-3y)=13 4（x+y）+3（x-3y)=1
2(x+y)-(x-3y)=13 4（x+y）+3（x-3y)=1
解方程组 2(x+y)-(x-3y)=13 4（x+y）+3（x-3y)=1

2(x+y)-(x-3y)=13 4（x+y）+3（x-3y)=1解方程组 2(x+y)-(x-3y)=13 4（x+y）+3（x-3y)=1
令a=x+y
b=x-3y
2a-b=13 (1)
4a+3b=1 (2)
(1)×3+(2)
6a+4a=39+1
a=4
b=2a-13=-5
所以
x+y=4
x-3y=-5
相减
4y=9
y=9/4
x=4-y=7/4

2x+2y-x+3y=13
x+5y=13....................1)
4x+4y+3x-9y=1
7x-5y=1....................2)
1)+2)得：
8x=14,x=7/4
代入1）：
5y=13-7/4=45/4
y=9/4

把式子化简，合并相同的项，第一个式子为x+5y=13,第二个式子为7x-5y=1
然后第一个式子乘以7，变为7x+35y=91.再和第二个式子相减。得到y=9/4,x=7/4

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