If the sum of the first n terms of an arithmetic progression is ${\mathrm{S}}_n=4n-n^2$, then can you find the value of the first term? Is this ${\mathrm{S}}_1$? What is the sum of first two terms of this arithmetic progression? What is the second term? In this way find the third the fourth, the fifteenth and the $n^{\mathrm{th}}$ terms.

Find the sum of the following Arithmetic Progression:

$9, 12, 15, ......$ up to $16$ terms.

Find the sum of the following Arithmetic Progression:

$8, 3, -2, ......$ up to $22$ terms.

Find the sum of the following Arithmetic Progression:

$0.6, 1.7, 2.8, ......$ up to $100$ terms.

Find the sum of the following Arithmetic Progression:

$\frac{1}{15}, \frac{1}{12}, \frac{1}{10}, ......$ up to $11$ terms.

Find the value of $k$ if sum of the following Arithmetic Progression: $\frac{n^2+1}{n}, n, \frac{n^2-1}{n}, ......$ up to $20$ terms is $\frac{10(2n^2-k)}{n}$

Find the sum of the following Arithmetic Progression:

$\left(1-\frac{1}{n}\right), \left(1-\frac{2}{n}\right), \left(1-\frac{3}{n}\right), ......$ up to $n$ terms.