Originally Posted by
nugiboy Hi im a bit confused on this question.

The first 3 terms of an arithmetical progression are 7, 5.9 and 4.8.
Find
(i) the common difference
(ii) the smallest value of n for which the sum to n terms is negative

I worked out the common difference to be 1.1, which i know is right. Im just a bit stuck on part 2. I know the formula for the sum of terms in an arethmetic series.
Sum of n = n/2 (2a + (n1)d)
n = term ie:5th term
a = first term, which in this case is 7
d = common difference which is 1.1
I tried to solve it by making Sum of n< 0
I get a wierd answer. Can someone show me where ive gone wrong.