If $$1.3 + {2.3^2} + {3.3^3} + ... + n{.3^n} = {{(2n - 1){3^a} + b} \over 4}$$, then a and b are respectively.

If $${S_n} = nP + {{n(n - 1)Q} \over 2}$$, where, S_n denotes the sum of the first n terms of an AP, then the common difference is

What is the sum of the series 0.3 + 0.33 + 0.333 + ..... n terms?

If the sum of m terms of an AP is n and the sum of n terms is m, then the sum of (m + n) terms is