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If \(S_n\) denotes the sum of the first \(n\) natural numbers, which of the following statements is always true for every natural number \(n\)?

Answer and explanation

Correct answer: \(8S_n+1\) is the square of an odd natural number.

Using \(S_n=\frac{n(n+1)}{2}\), we get \(8S_n+1=4n(n+1)+1=(2n+1)^2\), the square of an odd number. Option B is not a square for every \(n\). Exam tip: remember the \(8S_n+1\) property of triangular numbers.

Related tags

Natural NumbersTriangular NumbersSequencesProgressionsNumber PropertiesAlgebraic IdentitiesSum Of First N Natural NumbersSequences And Progressions

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