Pattern Problem 3
Level
pending
Find the 20th number in the following sequence:
-23, -9, 11, 37, 69, 107, ...
The answer is 1269.
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T(n) = n × (3n + 5) - 31
T(20) = 20 × (3 × 20 + 5) - 31
= 20 × 65 - 31
= 1300 - 31
= 1269