2/9-6/13×2/9
=2/9x(1-6/13)
=2/9x7/13
=12/117

36×（17/18-2/9）
=36x17/18-36x2/9
=34-8
=26

4/13×4/7-3/7×4/13
=4/13x(4/7-3/7)
=4/13x1/7
=4/91