Question

in an AP, the sum of first 10 terms is equal to the sum of first 20 terms,then find the sum of first 30 terms

Answer 1

sum of first ten terms=
10/2(2a+(10-1)*d)
sum of first twenty terms=
20/2(2a+(20-1)*d)
solving both we get
5(2a+9d)=10a+45d and
10(2a+19d)=20a+190d
on solving the above solution we can get a,first term and d,common difference. from this we can take out the sum of 20 terms