7 th of ap is 40 then the sum of first 13 term is 25
the steps of the sum:
a be the first term and d be the common difference
So the 7th term of the A.P. is 40=> a+6d =40.
Sum of first 13 terms = 13/2(a +(a+12d))= 13/2(2a+12d) =13/2(2)(a+6d) =13(a+6d) =13(40)=520 .
