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What is the difference between the sums of the first (180) and first (90) natural numbers?

Answer and explanation

Correct answer: 12195

The sum of the first n natural numbers is \(S_n=\frac{n(n+1)}{2}\). Thus, \(S_{180}=\frac{180\times181}{2}=16290\) and \(S_{90}=\frac{90\times91}{2}=4095\). Hence, the difference is \(16290-4095=12195\). It is also the sum of the integers from 91 to 180; a value such as 12295 does not result from the formula or subtraction. Exam tip: write such a difference as \(S_{180}-S_{90}\).

Related tags

Sequences And ProgressionsNatural NumbersSum FormulaArithmetic SeriesNumber SeriesSum Of First N Natural NumbersMathematicsClass 9 Mcq

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