Question

the n term of an ap is given by (-4n+15) find the sum of first 20 terms of this ap

Answer 1

An=a+(n-1)d
=-4n+15
a1=11,a2=7,a3=3
d=a2-a1=7-11=-4
Sn=n/2*2a+(n-1)*d=20/2*2*11+20-1*-4
=220-76
=144...