Question

the Nth term of an AP is given by (-4n+15). find the sum of first 20 terms of this A.P.

Answer 1

Tn=-4n+15
T1=-4(1)+15
=11=a.
T2=-4(2)+15
=7
T2-T1=11-7=4=d
Sn=n/2(2a+(n-1)d)
S20=20/2(2(11)+(20-1)7)
=1550

Answer 2

a1=-4*1+15=11
a20=-4*20+15=-65
n=20
Sn=n/2(a+an)
=20/2(11+(-65))
=10(-54)
=-540