Question

Hi how many terms of an ap - 15 - 13 - 11 to make the sum -55

Answer 1

Here, a=T1= -15,
And, d=T2-T1= -13-(-15)= -13+15=2
Now, Sn= n/2(2a+(n-1)d)
-55=n/2(2(-15)+(n-1)2)
-55(2)=n(-30+2n-2)
-110=n(2n-32)
-110=2n^2-32n
2n^2-32n+110=0
n^2-16n+55=0
n^2-5n-11n+55=0
n(n-5)-11(n-5)=0
(n-5)(n-11)=0
n-5=0 OR n-11=0
n=5 OR n=11
Therefore, n=11.

HOPE THIS HELPS.

Answer 2

plz refer to this attachment