5/13×7/9+7/13×4/9=7/13×5/9+7/13×4/9=7/13×（5/9+4/9）=7/13×1=7/13
5/13*7/9+7/13*4/9=7/13*5/9+7/13*4/9=7/13*（5/9+4/9）=7/13*1=7/13
5/13x7/9+7/13x4/9=7/9x（5/13+4/13）=7/9x9/13=7/13
