Adding irrationals as fractions!

Algebra Level 2

1 1 + 2 + 1 2 + 3 + 1 3 + 4 + + 1 8 + 9 = ? \dfrac1{1+\sqrt2} + \dfrac1{\sqrt2+\sqrt3} + \dfrac1{\sqrt3+\sqrt4} + \cdots + \dfrac1{\sqrt8+\sqrt9} = \, ?

2 3 4 5

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2 solutions

Andrew Ellinor
Jan 27, 2016

Start by rationalizing each denominator. That is, for the denominator n + n + 1 \sqrt{n} + \sqrt{n + 1} , multiply the top and bottom of that fraction by n n + 1 . \sqrt{n} - \sqrt{n + 1}. In doing so, the given expression becomes

1 2 1 + 2 3 1 + 3 4 1 + + 8 9 1 , \frac{\sqrt{1} - \sqrt{2}}{-1} + \frac{\sqrt{2} - \sqrt{3}}{-1} + \frac{\sqrt{3} - \sqrt{4}}{-1} + \cdots + \frac{\sqrt{8} - \sqrt{9}}{-1},

which simplifies to

( 2 1 ) + ( 3 2 ) + ( 4 3 ) + + ( 9 8 ) = 9 1 = 2. (\sqrt{2} - \sqrt{1}) + (\sqrt{3} - \sqrt{2}) + (\sqrt{4} - \sqrt{3}) + \cdots + (\sqrt{9} - \sqrt{8}) = \sqrt{9} - \sqrt{1} = 2.

Puneet Pinku
Jan 15, 2016

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