Find the sum of the radii of the fcircles, which are respectively inscribed and circumscribed about the a regular polygon of n sides.

Find the sum of : the first n positive integers

Find the sum of the product of every pair of the first n of natural number

Find the sum to n terms of the series whose n^("th") term is n (n+3).

Find the sum to n terms of the series in whose n^(th) terms is given by n^2+2^n

Find the sum to n terms of the series in whose n^(th) terms is given by (2n-1)^2

Find the number of triangles whose angular points are at the angular points of a given polygon of n sides, but none of whose sides are the sides of the polygon.

The radii of two circles are 8 cm and 6 cm respectively. Find the radius of the circle having area equal to the sum of the areas of the two circles.

Find the sum to n terms of the series in whose n^(th) terms is given by n(n+1)(n+4)

Find the sum of the series whose n^(th) term is as following: n^(3)-3n

Find the sum to n terms of the sequence, 8, 88, 888, 8888… .