Tribute to my first problem

Algebra Level 2

Is the following inequality correct? 1 51 + 1 52 + 1 53 + 1 200 > 1 \begin{aligned}\frac{1}{51}+\frac{1}{52}+\frac{1}{53}+\cdots\frac{1}{200}>1\end{aligned}

yes no

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

3 solutions

Sumukh Bansal
Oct 27, 2017

We form the summations k = 51 200 1 k = k = 51 100 1 k + k = 101 150 1 k + k = 151 200 1 k > 1 2 + 1 3 + 1 4 = 6 + 4 + 3 12 = 13 12 > 1 \displaystyle\sum_{k=51}^{200} \dfrac{1}{k} = \sum_{k=51}^{100} \dfrac{1}{k} + \sum_{k=101}^{150} \dfrac{1}{k} + \sum_{k=151}^{200} \dfrac{1}{k} \gt \dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{4} = \dfrac{6 + 4 + 3}{12} = \dfrac{13}{12} \gt 1 .

So 1 51 + 1 52 + 1 53 + 1 200 > 1 \begin{aligned}\frac{1}{51}+\frac{1}{52}+\frac{1}{53}+\cdots\frac{1}{200}>1\end{aligned} is greater than 1 \boxed{\text{1}}

Naren Bhandari
Oct 21, 2017

1 51 + 1 52 + 1 53 + + 1 200 \begin{aligned}\frac{1}{51}+\frac{1}{52}+\frac{1}{53}+\cdots+\frac{1}{200}\end{aligned}

Note that n < n + 1 < n + 2 < n + k 1 n > 1 n + 1 + 1 n + 2 > > 1 n + k \begin{aligned} & n < n+1< n+2 < \cdots \ n+k \\& \implies \frac{1}{n} > \frac{1}{n+1}+\frac{1}{n+2} > \cdots > \frac{1}{n+k}\end{aligned} for n N n\mathbb\in N and k = 1 , 2 , 3 , k=1,2,3,\cdots S = 1 51 + 1 52 + 1 53 + + 1 200 = ( 1 51 > 1 100 + 1 52 > 1 100 + 1 53 > 1 100 + 1 99 > 1 100 + 1 100 ) + ( 1 101 > 1 200 + 1 102 > 1 200 + 1 103 > 1 200 + + 1 200 ) = n = 51 100 1 n > 50 × 1 100 + k = 101 200 1 k > 100 × 1 200 > 1 2 + 1 2 > 1 \begin{aligned} S & = \frac{1}{51}+\frac{1}{52}+\frac{1}{53} +\cdots+\frac{1}{200} \\& = \left(\frac{1}{51} > \frac{1}{100}+\frac{1}{52}> \frac{1}{100}+\frac{1}{53} > \frac{1}{100} \cdots +\frac{1}{99}> \frac{1}{100}+\frac{1}{100}\right)+\left(\frac{1}{101}> \frac{1}{200}+\frac{1}{102}> \frac{1}{200}+\frac{1}{103}>\frac{1}{200} +\cdots+\frac{1}{200}\right) \\&= \displaystyle\sum_{n=51}^{100}\frac{1}{n}>50\times \frac{1}{100} + \displaystyle\sum_{k=101}^{200}\frac{1}{k} > 100\times\frac{1}{200} \\& > \frac{1}{2}+\frac{1}{2} > 1 \end{aligned} Thus the answer is yes \text{yes} .

Noting that k = 51 100 1 k > 50 × 1 100 = 1 2 , k = 101 150 1 k > 50 × 1 150 = 1 3 \displaystyle\sum_{k=51}^{100} \dfrac{1}{k} \gt 50 \times \dfrac{1}{100} = \dfrac{1}{2}, \sum_{k=101}^{150} \dfrac{1}{k} \gt 50 \times \dfrac{1}{150} = \dfrac{1}{3} and k = 151 200 1 k > 50 × 1 200 = 1 4 \displaystyle\sum_{k=151}^{200} \dfrac{1}{k} \gt 50 \times \dfrac{1}{200} = \dfrac{1}{4} ,

we find that k = 51 200 1 k = k = 51 100 1 k + k = 101 150 1 k + k = 151 200 1 k > 1 2 + 1 3 + 1 4 = 6 + 4 + 3 12 = 13 12 > 1 \displaystyle\sum_{k=51}^{200} \dfrac{1}{k} = \sum_{k=51}^{100} \dfrac{1}{k} + \sum_{k=101}^{150} \dfrac{1}{k} + \sum_{k=151}^{200} \dfrac{1}{k} \gt \dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{4} = \dfrac{6 + 4 + 3}{12} = \dfrac{13}{12} \gt 1 . The answer is therefore yes \boxed{\text{yes}} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...