CBSE Class 10 Answered
Please find the sum.
Asked by sunil_0010 | 01 Jul, 2009, 10:32: PM
Expert Answer
we see that the first term=a=54
common diff =d=-3
let the sum of n terms is 513
so using the formula we get,
n/2[2a+(n-1)d]=513
putting the values of a and d,we get a quadraticc equation, on factorising that
we get,
(n-18)(n-19)=0
so
n=18
or
n=19
so if the number of terms is 18 or 19, we get the sum as zero.
now, for the double answer
since the 18th term
=a+17d
=54+17(-3)
=3
since the common diff is -3,
the next term i.e.the 19th term will be
3+(-3)=0
so whethter you take the sum of 18 terms or 19 terms , it will still be 513
as the 19th term is zero.
Answered by | 02 Jul, 2009, 09:37: AM
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