a)
1/2+1/3= 3/6+ 2/6 = 5/6
1/3+1/4= 4/12+ 3/12= 7/12
1/4+1/5= 5/20+ 4/20= 9/20
b)
1/5+2/3= 3/15+ 10/15= 13/15
1/6+3/4= 4/24+ 18/24= 22/24
3/7+2/5= 15/35+ 14/35= 29/35
