Please find the sum.

Asked by sunil_0010 | 1st 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  | 2nd Jul, 2009, 09:37: AM

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