Number system important formulae

1. When sum and difference of two numbers (X and Y) are given, then

X = (sum + difference)/2

Y = (sum + difference)/2

2. Sum of first n odd numbers is n2

3. Sum of first n even numbers n(n+1)

4. Difference between two digits of two digit number is = (Difference in original and interchanged number)/9

5. Sum of squares of first n natural numbers is n(n+1)(2n+1)/6

6. Sum of cubes of first n natural numbers is [n(n+1)/2]2