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

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

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(Rock)

> 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

???

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