Question

find the sum of arithmetic sequence series 7+14+21.....+140​

Answer 1

Answer:

Step-by-step explanation:

Here a=7, d= 14-7=7, l (last or nth term) = 140

We know Tn = a+(n-1)d

         140 = 7 + (n-1)7

            20 = 1+(n-1)1

             n = 20

Now, Sn = n/2(a+l)

               = 20/2(7+140)

               =10*147  = 1470

Answer 2

Answer:

=n/2(x1+xn)

=20/2(7+140)

=10×147

=1470