Question

the common difference.
5. If the sums of the first 8 and 19 terms of an A.P. are 64 and 361 respectively, find (i) the common
difference and (ii) the sum of n terms of the series.
number of terms of the series 21 18 15 12
which must be taken to give a sum of
ISCU​

Answer 1

Step-by-step explanation:

(i) given

a+7d=64-----1

a+18d=361-----2

by subtracting 1 from 2nd we get

-11d= -297

d=27

putting value of D in ---1

we get

a+7×27=64

a=64-189

a= -125

ii)

Sn=n/2(2a+(n-1) d

=n/2(2×-125)+(n-1)27