Question

For the A.P: 10, 15, 20,....,195; find: the sums of all its terms.​

Answer 1

an= a+(n-1)d

195 =10+(n-1)5 , 195-10=(n-1)5 , 185=(n-1)5 ,185÷5= n-1 ,37=n-1 ,37+1=n, 38= n. Sn = n÷2(a+l), Sn= 19(10+195), Sn=19(205), Sn=3895