Question

how many terms of the AP 24,21,18.. must be taken so that their sum is 18​

Answer 1

sn=n/2(2a+(n-1)d)

18=n/2(2×24+(n-1)-3

36=n(48+(n-1)-3)

36=n(48-3n+3)

36=n(51-3n)

36=51n-3nn

3nn-51n+36=0

3(nn-17n+12)=0

nn-17n+12=0