Question

The sum of first five terms of an ap and the sum of srven terms of the same ap is 167.If the sum of the first ten terms of an ap is 235 fins the sum of its twenty term

Answer 1

s5+s7=167

= 5/2(2a+(5-1)d)+7/2(2a+(7-1)d)=167-----eqn.1

s10=235

10/2(2a+(10-1)d)=235

2a+9d=47---------eqn.2

solving both the eqn. we get a=1 and d=5

s20=20/2(2(1)+(19)5

=10(97)

=970