Question

how many term of an AP series 63, 60, 57,...
be taken so that their sum is 693

Answer 1

Sn=n/2(2a+(n-1)d)
693=n/2(2×63-3n+3). (d= -3)
2×693=n(126+3-3n)
1386=129n-3n^2
3n^2-129n+1386=0
n^2-43n+462=0
n= (43+√(1849-1848))/2. and. (43 - √(1849-1848))/2
n=21 and 22..