1,x-y=2
xy=15 (x=5,y=3)
2,x+y=6
x-y=2 (4,2)
3,x+y=11,xy=30 (x=5,y=6,x=6,y=5)
4,x+y=13,xy=42 (x=6,7,y=7,6)
5,x+y=11,x-y=1 (x=6,y=5)
6,x+y=12,xy=35 (x=5,7,y=7,5)
7,x+y=5,xy=6 (x=2,3,y=3,2)
8,x-y=5,xy=36 (x=6,y=1)
9,x+y=10,xy=25 (x=y=5)
10,x+y=17,y-x=1 (x=8,y=9)
11,xy=2,x-y=1 (x=2,y=1)
12,x+y=3,xy=2 (x=1,2,y=2,1)
13,x+y=12,xy=11 (x=1,11,y=11,1)
14,x-y=8,xy=9 (x=9,y=1)
15,x+y=4,x-y=2 (x=3,y=1)
16,x+y=3,xy=0 (x=0,3,y=3,0)
17,x-y=5,xy=6 (x=6,y=1)
18,y-x=3,xy=28 (x=4,y=7)
19,y-x=2,xy=24 (x=4,y=6)
20,x+y=9,x-y=1,(x=5,y=4)http://wenku.baidu.com/search?word=%B6%FE%D4%AA%D2%BB%B4%CE%B7%BD%B3%CC%D7%E9%B5%C4%CC%E2&lm=0&od=0