MEDAL (PLEASE PLEASE HELP) The sequence 15, 21, 27, 33, 39, ...., 75 has 11 terms.

Question

Evaluate the related series. A) 420 B) 495 C) 210 D) 480

Answer 1

The sequence is in arithmetic progression:

Calculate the common difference;

d = 21-15 =6

Number of terms n =11

First term a = 15

Calculate the sum of terms.

Sn =(n/2)(2a+(n-1)d)

     = (11/)(2(15)+(11-1)(6))

      =(11/2)(30+60)

     =(11/2)(90)

     =(11)(45)

    =495

Option B is correct.