Why is it necessary to have common denominators when adding fractions?

It makes it a whole lot easier and more correct!!

1= Numerator/5:denominator    2=numerator/7:denominator

1. Write out your problem 1/5+2/7
2. Multiply denominators 7x5=35
3. add how many times each denominator goes into multiplication
answer 7+1/35 + 5+2/35
4. Add 8/35 + 7/35
5. Simplify (if necessary)15/35
6. Answer 3/7

3=numerator and 7=denominator.

I hope this helps! :)