Question

Find the sum of 25th term of an ap whose end term is given by tn is equal to 7 minus 3 and

Answer 1

tn=7-3n
If n=1,
7-3(1)=7-3=4(a)
If n=2,
7-3(2)=7-6=1(a2)
a2-a1=1-4=-3(d)
Sn=n/2(2a+(n-1)d)
S25=25/2(2×4+24×-3)
S25=25(8-72)
= 25/2×-64
=25×-32=-800