An arithmetic series has first term a and common difference d.

The sum of the first 31 terms of the series is 310.

a)Show that a + 15d = 10

> S31 = 31/2 ( 2a + (31-1) d)

> 31( a + 15d ) = 310

> a + 15d = 310/31; a + 15d = 10

b)Given also that the 21st term is twise the 16th term, find the value of d.

???

c)the nth term of the series is un. Given that

K

Sum un = 0, find the value of K.

n = 1

???