Find Sn for each arithmetic series described

Question

1. a, = 3, an = 20, n -6 




5. a,=-80 , n =18 an = 120

 

 

Answer 1

1) In arithmetic series,  Sn = n/2 (a₁ + an)

where a₁ = 1st term, an = nth term = an = a₁ + (n-1)d, n = number of terms and Sn = sum of n terms.

     Sn = n/2 (a₁ + an)          where a₁=a= 3, n = 6 and an = 20

          = 6/2 (3 + 20)

          = 3 (23)

          = 69

Therefore Sum of 6 terms is 69.

5)   Sn = n/2 (a₁ + an)          where a₁=a= -80, n = 18 and an = 120

           = 18/2 (-80 + 120)

           = 9 (40)

           = 360

Therefore Sum of 18 terms is 360.