Question

What is the sum of a 10–term arithmetic sequence where the first term is 3 and the last term is 75?

Answer 1

\[\S _{10}=\frac{1}{2}(10)(3+75))=5(78)=390\]

Answer 2

first term is 3 ,so a=3
and last term is 75 ,so L=75
_________
sn=n\2(a+L) ,s10=10\2(3+75)=390