Question

How many terms of the AP : -15,-13,-11....... are needed to make the sum -55?

Answer 1

Answer:

here is your answer

I hope it will help u

Answer 2

Answer:

heya mate there's your answer

given series -15,-13,-11................

let the first term a1=-15 ,a2=-13

and d be the common difference

we know that d=a2-a1

d=-13-(-15)

d=-13+15

d=2

Sum of n terms in an AP is

Sn=n/2[2a1+(n-1)d]

given sum of some terms is -55

let Sn=-55 then ,from formula we get,

Sn=n/2[2a1+(n-1)d]

-55=n/2[2(-15)+(n-1)2]

-55×2=n(2)[-15+n-1]

-55=n[-16+n]

-55=-16n+n^2

n^2-16n+55=0

n^2-11n-5n+55=0

n(n-11)-5(n-11)=0

(n-11)(n-5)=0

n=11 and n=5

hope it helps you

Thank you