Вычислите: 2/9+7/9; 2/3-5/9; 1/6+2/9; 3/5-1/7; (7/15-1/6)+2/5

2/9 + 7/9 = 9/9 = 1 

2/3 - 5/9 = 6/9 - 5/9 = 1/9 

1/6 + 2/9 = 3/18 + 4/18 = 7/18 

3/5 - 1/7 = 21/35 - 5/35 = 16/35 

( 7/15 - 1/6 ) + 2/5 = ( 14/30 - 5/30) + 12/30  = 9/30 + 12/30 = 21/30