Ответ:

a) 3/5+2/7= 7*3 7*5= 21/35 5*2 5*7=10/35 21/35+10/35=31/35

b) 3/4+1/7= 7*3 7*4= 21/28 4*1 4*7= 4/28 21/28+4/28=25/28

c) 4/9+2/5= 5*4 5*9= 20/45 9*2 9*5= 18/45 20/45+18/45= 38/45

d) 5/11+2/9=9*5 9*11= 45/99 11*2 11*9=22/99 45/99+22/99=67/99