Think before choose

Algebra Level 3

16 17 + 17 18 + 18 19 + + 32 33 16 \begin{array}{c}\dfrac{16}{17} + \dfrac{17}{18} + \dfrac{18}{19} + \cdots + \dfrac{32}{33} & \huge \square & 16 \end{array}

What sign < < , = = , or > > should be in the box?


This is part of the series: It's easy, believe me!

= = < < > >

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2 solutions

Chew-Seong Cheong
Aug 18, 2017

S = 16 17 + 17 18 + 18 19 + + 32 33 = 17 1 17 + 18 1 18 + 19 1 19 + + 33 1 33 = 17 ( 1 17 + 1 18 + 1 19 + + 1 33 ) > 17 ( 1 17 + 1 17 + 1 17 + + 1 17 ) > 17 1 = 16 \begin{aligned} S & = \frac {16}{17} + \frac {17}{18} + \frac {18}{19} + \cdots + \frac {32}{33} \\ & = \frac {17-1}{17} + \frac {18-1}{18} + \frac {19-1}{19} + \cdots + \frac {33-1}{33} \\ & = 17 - \color{#3D99F6} \left(\frac {1}{17} + \frac {1}{18} + \frac {1}{19} + \cdots + \frac {1}{33} \right) \\ & \ {\color{#D61F06}>} \ 17 - \color{#3D99F6} \left(\frac {1}{17} + \frac {1}{17} + \frac {1}{17} + \cdots + \frac {1}{17} \right) \\ & > 17 - {\color{#3D99F6} 1} = 16 \end{aligned}

S > 16 \implies S \ \boxed{>} \ 16

Great solution. Still wonder why a lot of people get tricked.

Thành Đạt Lê - 3 years, 9 months ago

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I don't know.

Chew-Seong Cheong - 3 years, 9 months ago

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I think it's because they think there are only 16 terms on the LHS.

Calvin Lin Staff - 3 years, 9 months ago

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@Calvin Lin Oh, I think so too.

Chew-Seong Cheong - 3 years, 9 months ago

Maybe they solve this problem this way: 16 17 < 1 \dfrac{16}{17} < 1 17 18 < 1 \dfrac{17}{18} < 1 18 19 < 1 \dfrac{18}{19} < 1 . . . ... 32 33 < 1 \dfrac{32}{33} < 1 So that: 16 17 + 17 18 + 18 19 + . . . + 32 33 < 16 \dfrac{16}{17} + \dfrac{17}{18} + \dfrac{18}{19} + ... + \dfrac{32}{33} < 16 I made this question just to see if people can get tricked. And it seems like some people do get tricked.

Thành Đạt Lê - 3 years, 9 months ago
Arjen Vreugdenhil
Aug 25, 2017

There are seventeen terms here. The first one is equal to 16 / 17 16/17 ; the other ones are all greater.

(Consider, e.g. that 17 18 = 1 1 18 > 1 1 17 = 16 17 \frac{17}{18} = 1 - \frac1{18} > 1 - \frac1{17} = \frac{16}{17} .)

Thus we have 16 17 + 17 18 + + 32 33 > 16 17 + 16 17 + 17 × = 17 16 17 = 16. \frac{16}{17} + \frac{17}{18} + \cdots + \frac{32}{33} \boxed{>} \underbrace{\frac{16}{17} + \frac{16}{17} + \cdots}_{17\times} = 17\cdot \frac{16}{17} = 16.

I liked this answer. Its so simple and clever!

Abha Vishwakarma - 2 years, 9 months ago

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