8/9 - x = 5/6
x = 8/9 - 5/6
x = 16/18 - 15/18
x = 1/18
3/17 + x = 5/17
x = 5/17 - 3/17
x = 2/17
x - 4/39 = 5/13
x = 5/13 + 4/39
x = 15/39 + 4/39
x = 19/39
4 - x = 2/7
x = 3+ 7/7 - 2/7
x = 3+ 5/7
x = 3又5/7
7/8 - 7/9
= 63/72 - 56/72
= 7/72
1/2 + 1/3 + 1/6
= 3/6 + 2/6 + 1/6
= 6/6
= 1