Question

find the sum of the multiples of 7 lying between 500 and 900

Answer 1

504,511.....889
sn=n/2(a+an)
sn=7/2(504+889)
=7/2(1393)
=

Answer 2

Sn=n/2[2a+(n-1)d]
S7=7/2[2*500+(7-1)900]
=7/2*1000+5400
=7*500+5400
=3500+5400
=8900ans