Find the sum of the series
10+1+8+2+6+3+....+10(-10)
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in arithmetic progression
an = a1 + (n-1)d
s = n (a1 + an)/2
in this question,
a1 = 10
d = - 1/3
an will be
an= 10 -1/3 (n-1)
s= n (10 + 10 - 1/3 (n-1)) / 2 = 155
n (20 - 1/3 n + 1/3) = 310
20 n - 1/3 n2 + 1/3 n = 310
60 n - n2 + n = 930
n2 - 61 n + 930 = 0
(n - 30) (n -31) = 0
n = 30 or n = 31
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