alleryThe 6th triangular number is:
A15
B21
C28
D36
Answer & Solution
Correct answer: B. 21
Triangular numbers: 1 = 1, 3 = 1+2, 6 = 1+2+3, 10 = 1+2+3+4, 15 = 1+2+3+4+5, 21 = 1+2+3+4+5+6. So T₆ = 21. (Formula Tₙ = n(n+1)/2 gives T₆ = 6·7/2 = 21.)
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