Question

Find the sum of the first 20 terms of the arithmetic sequence 4, 10, 16, 22....

Answer 1

n = 20

d = 6

a = 4

L = a + (n - 1)*d

L = 4 + (20 - 1)*6

L = 4 + 19*6

L = 118

Sum = (a + L)*n/2

Sum = (4 + 118) * 20/2

Sum = 122 * 10

Sum = 1220