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Complete the sequence in the series, 1, 3, 6, 10, 15, ____
The correct answer is option [E] 3 - 1 = 2; 6 - 3 = 3; 10 - 6 = 4; 15 - 10 = 5; The differences between two
consecutive numbers are 2, 3, 4, 5, 6, ... So, 21 - 15 = 6. Alternatively, 1 + 2 = 3; 1 + 2 + 3 = 6; 1 + 2 + 3 + 4 = 10;
1 + 2 + 3 + 4 + 5 = 15; 1 + 2 + 3 + 4 + 5 + 6 = 21; The nth term in the sequence is given by n (n + 1)/2. The
numbers are often referred to as triangular numbers.
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