Advanced MathHardNumber of terms from a triangular-number formula
Advanced Math practice question
The sum of the first n positive integers is given by n(n + 1)/2. For what value of n is this sum equal to 78?
Answer: 12
n(n + 1)/2 = 78 → n² + n − 156 = 0 → (n − 12)(n + 13) = 0 → n = 12.
Common wrong answers
- If you answered 13
- 13 gives 13(14)/2 = 91.
- If you answered 39
- 39 is 78 ÷ 2, ignoring the quadratic.
- If you answered -13
- −13 is the rejected negative root.
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