Question

Find the sum of 1st 25 term of an ap whose nth term is 1-4n

Answer 1

tn=1-4n,s25=?
put n=1
t1=-3
therefore a=-3
now put n=2
t2=-7
d=t2-t1=-7-(-3)
d=-4
We know that,
sn=n/2[2a+(n-1)d]
S25=25/2[2(-3)+(25-1)-4]
= 25/2[-6-96]
=25/2(-102)
=25(-51)
S25 =-1275
Therefore the sum of first 25 terms is -1275.
HOPE THIS HELPS!!!