alleryThe fifth triangular number, 15, equals:
A1+3+5+7+9
B2+4+6+8+10
C5+5+5+5+5
D1+2+3+4+5
Answer & Solution
Correct answer: D. 1+2+3+4+5
1. Each triangular number sums the natural numbers up to it.
2. The fifth one adds the first five natural numbers.
3. So 15 is 1 + 2 + 3 + 4 + 5.
4. The answer is 1+2+3+4+5.
_Source: syllabus Mathematics, Ch 8 'Predicting What Comes Next: Exploring Sequences and Progressions'_
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