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Under which condition will the sum of the first n natural numbers be an even number?

Answer and explanation

Correct answer: n is a multiple of 4 or leaves remainder 3 when divided by 4

The sum of the first n natural numbers is n(n+1)/2. One of n and n+1 is even; after dividing by 2, the remaining factor must also be even. Thus the sum is even when n ≡ 0 or 3 (mod 4). Exam tip: check n modulo 4 directly.

Related tags

Natural NumbersSum FormulaParityModulo ArithmeticSequences And ProgressionsGrade 9 MathematicsSum Of First N Natural NumbersMathematics

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