CBSE Class 10 Answered
Please solve the sum.
Asked by sunil_0010 | 01 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 | 01 Jul, 2009, 10:54: PM
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