Question

find the sum 3 +12+21+30+......+930

Answer 1

a1=3, a2=12, last term(l) or an=930

d=a2-a1=12-3=9

an=a+(n-1)d

930=3+(n-1)9

930-3=(n-1)9

927=(n-1)9

103=n-1

n=104

Now using Sn formula

Sn= n /2(a+l)

S104=104/2(3+930)

=52(933)

=48516