Please solve the sum.

Asked by sunil_0010 | 1st Jul, 2009, 10:43: PM

Expert Answer:

let a be the first term and d be the common diff

5th term=a+4d

9th term=a+8d

so

5th term +9th term

=2a+12d=8

so,

a+6d=4..(i)

next their product is 15

(a+4d)(a+8d)=15..(ii)

put

a=4-6d in (ii)

so we get,

d=plus or minus (1/2)

then find a using (i)

once you have the values of a and d

the sum of first 20 terms can be found using the formula

=20/2[2a+19d]

Answered by  | 1st Jul, 2009, 10:54: PM

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