Question

how many terms of the series 24+20+16+..... gives the sum 72 ?

Answer 1

4 terms are in this series...

24+20+16+12=72

Answer 2

a=24

d= - 4

Sn=72

Sn=n/2(2a+(n-1)d)
72=n/2(2×24+(n-1)-4)
72×2=n(48+(n-1)-4)
144=n(48-4n+4)
144=n(52-4n)
144=52n-4n^2

4n^2-52n+144=0
n^2-13n+36=0
n^2-9n-4n+36=0
(n-9)(n-4)=0
n=4
n=9