Question

find the sum of first 19 terms of the AP whose sn = 7-4n

Answer 1

an=7-4n
Put n=1
a1=7-4*1=3
Put n=2
a2=7-4*2=-1
d=-4
Sn=19/2[2*3+(19-1)-4]
=19/2(6-75)
=19/2*-66
=-627

Answer 2

sn-7-4n the 19 terms of the Ap is 1975