The 5th and 6th triangular numbers are 15 and 21. Their sum equals a perfect square. What is it?
- A.30
- B.36 ✓
- C.42
- D.49
Explanation: 15 + 21 = 36 = 6². (In general, the sum of two consecutive triangular numbers is always a perfect square.)
Class 6 IMO Mathematics Olympiad practice · Algebraic Thinking · Patterns in Mathematics
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