Question

find sum of allmultiples of 3, lying between 200 and 300

Answer 1

answer is 8127. sequence we will get 201 + 204+-----------+297 according to Ap s=n/2(a+I) here. n=33 a=201 I=297 substitute we will get answer. 8127

Answer 2

Let be all multiple lies between 200 and 300.
Now, 201,204,207,......,300.

Now,in AP, a=201 and d=204-201=3 and an=300

An=a+(n-1)d
300=201+(n-1)3
99=3-3n
102=3n
n=34


Now, Sn=n/2(2a+(n-1)d)
= 34/2(2*201+(34-1)3)
=17 (402+99)
=17 (501)
=8517
Hence, S300=8517