Question

what is the sum of 3+6+9+12+15+.............+300​

Answer 1

Answer is 15150

Explanation: first term is 3 difference is 6-3=3 so difference is 3 last term is 300

nth term =a+(n-1)d

300=3+(n-1)3

297/3=(n-1)

n=100

Sn=n/2

(2a+(n-1)d)

50(6+99(3))=15150

Answer 2

In the given AP,
a=3
d=3
l=300
l=a+(n-1)d
300=3+(n-1)3
297=(n-1)3
99=n-1


n=100

So,
Sum of first n terms are,
Sn=n/2[2a+(n-1)d]
Substitute the values of a,d and n,
=50[6+99(3)]
=50(6+297)
=50(303)
=15150