The sum of the series $$3 - 1 + {1 \over 3} - {1 \over 9} + .......$$ is equal to

Let T_r be the r^th term of an AP for r = 1, 2, 3, ...... If for some distinct positive integers m and n we have T_m = 1 / n and T_n = 1 / m, then what is T_mn equal to?

If the second term of a GP is 2 and the sum of its infinite terms is 8, then the GP is

If a, b, c are in AP or GP or HP, then $${{a - b} \over {b - c}}$$ is equal to