The sum of how many terms of the series 6 + 12 + 18 + 24 + ... is 1800

1.	The sum of how many terms of the series 6 + 12 + 18 + 24 + ... is 1800 ?
Answer» This is an A.P. in which a = 6, d = 6 and Sn = 1800 Then, n [2a + (n - 1)d] = 1800 2 n [2 x 6 + (n - 1) x 6] = 1800 2 3n (n + 1) = 1800 n(n + 1) = 600 n2 + n - 600 = 0 n2 + 25n - 24n - 600 = 0 n(n + 25) - 24(n + 25) = 0 (n + 25)(n - 24) = 0 n = 24 Number of terms = 24.

Discussion