Answers

2015-01-27T17:12:07+05:30
Sum of n terms is 1800
first term=6
common difference =6
by formula, sum of n terms=n/2[2a+(n-1)d]
1800=n/2[12+(n-1)6]
3600=n[12+6n-6]
3600=6n+6n²
6n²+6n-3600=0
n²+n-600=0
n²+25n-24n-600=0
n(n+25)-24(n+25)=0
(n+25)(n-24)=0
so, n=24
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