The number sequence, - 15 * (5 / 6) of (n minus 1 power) + 1, when n is equal to what, the sum of the number sequence is the smallest

The number sequence, - 15 * (5 / 6) of (n minus 1 power) + 1, when n is equal to what, the sum of the number sequence is the smallest

Let - 15 * (5 / 6) ^ (n-1) + 1 ≥ 0, then (5 / 6) ^ (n-1) ≤ 1 / 15. Take logarithm on both sides of equation (5 / 6) ^ (n-1) = 1 / 15 to get n-1 = LG (1 / 15) / LG (5 / 6) = LG15 / (lg6-lg5). Substitute LG2 ≈ 0.3010, Lg3 ≈ 0.4771 to get n ≈ 15.9