What are rational numbers?
A number of the form 
, where a and b are integer and b is not equal to zero is called rational numbers.
Addition of Rational Numbers
If the denominators of the given rational numbers are co-prime, then their LCM is the product of the denominators.
In general, if p/q and r/s are two rational numbers and q and s are co-primes, then
Examples
Find the sum
(I) -3/9 and 5/7 (ii) -4/3,8/9 and -3/8
Solution: (i) Here, 9 and 7 are co-primes.
as we know:
= 8/21