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
Sum un = 0, find the value of K.
n = 1