Question

find the number of terms of an ap 63, 60, 57 so that their sum is 693 explain the double answer

Answer 1

63,60,57......
Sn=693 a=63 d=63-60=3
Sn=n/2(2a+(n-1)d)
693=n/2(2×63+(n-1)3
693=n/2(126+3n-3)
693=n/2(123+3n)
693×2=123n+3n^2
3n^2+123n-1389=0. Whole / by 3
n^2+41n-462=0