Question

The first and last term of an ap are 4 and 81respectively if the common differncr is 7 how many terms there are in the ap and what is there sum

Answer 1

a=4;d=7;an=81 an=a+(n-1)d;81=4+(n-1)7;77=(n-1)7;n-1=11;n=11;s12=12/2(4+81)=6×85=510