Find the positive integer solutions

In summary, a positive integer is a whole number greater than zero. To find the positive integer solutions to an equation, you can set the equation equal to zero, factor it, and find the factors that make the equation equal to zero. Multiple positive integer solutions can exist for an equation. To determine if a positive integer solution is correct, you can plug it back into the original equation. Not all equations have positive integer solutions, as some may have negative or irrational solutions or no real solutions at all.
  • #1
anemone
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Find all the positive integers $x$ such that $\dfrac{1}{4}<\dfrac{1}{x+1}+\dfrac{1}{x+2}+\dfrac{1}{x+3}<\dfrac{1}{2}$.
 
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  • #2
we have
$\dfrac{1}{x}+ \dfrac{1}{x+2}$
=$ \dfrac{2x+2}{x^2+2x}$
$\gt \dfrac{2x+2}{x^2+2x+1}$
$\gt \dfrac{2(x+1)}{(x+1)^2}$
$\gt \dfrac{2}{x+1}$
hence
$\dfrac{1}{x+1}+ \dfrac{1}{x+2}+ \dfrac{1}{x+3}\gt\dfrac{3}{x+2}$
further
$\dfrac{1}{x+1}+ \dfrac{1}{x+2}+ \dfrac{1}{x+3}\lt \dfrac{3}{x+1}$
so to get the upper bound of x we have
$\dfrac{3}{x+1} \gt \dfrac{1}{4}\gt \dfrac{3}{x+2}$
so $10\lt x \lt 11$
so $x\le10\cdots(1)$
so to get the lower bound of x we have
$\dfrac{3}{x+1} \gt \dfrac{1}{2}\gt \dfrac{3}{x+2}$
so $4\lt x\lt 5$
so $x\ge5\cdots(2)$
from (1) and (2) $ 5 \le x \le 10$
 
  • #3
kaliprasad said:
we have
$\dfrac{1}{x}+ \dfrac{1}{x+2}$
=$ \dfrac{2x+2}{x^2+2x}$
$\gt \dfrac{2x+2}{x^2+2x+1}$
$\gt \dfrac{2(x+1)}{(x+1)^2}$
$\gt \dfrac{2}{x+1}$
hence
$\dfrac{1}{x+1}+ \dfrac{1}{x+2}+ \dfrac{1}{x+3}\gt\dfrac{3}{x+2}$
further
$\dfrac{1}{x+1}+ \dfrac{1}{x+2}+ \dfrac{1}{x+3}\lt \dfrac{3}{x+1}$
so to get the upper bound of x we have
$\dfrac{3}{x+1} \gt \dfrac{1}{4}\gt \dfrac{3}{x+2}$
so $10\lt x \lt 11$
so $x\le10\cdots(1)$
so to get the lower bound of x we have
$\dfrac{3}{x+1} \gt \dfrac{1}{2}\gt \dfrac{3}{x+2}$
so $4\lt x\lt 5$
so $x\ge5\cdots(2)$
from (1) and (2) $ 5 \le x \le 10$

Well done, kaliprasad! And thanks for participating!
 

FAQ: Find the positive integer solutions

What is the definition of a positive integer?

A positive integer is a whole number greater than zero.

How do you find the positive integer solutions to an equation?

To find the positive integer solutions to an equation, you can start by setting the equation equal to zero and then factoring it. From there, you can find the factors that make the equation equal to zero and those will be the possible solutions.

Can there be more than one positive integer solution to an equation?

Yes, there can be multiple positive integer solutions to an equation. For example, the equation x^2 - 4 = 0 has two positive integer solutions: 2 and -2.

How do you know if a positive integer solution is the correct solution to an equation?

To determine if a positive integer solution is correct, you can plug it back into the original equation and see if it satisfies the equation. If it does, then it is a valid solution.

Can all equations have positive integer solutions?

No, not all equations have positive integer solutions. Some equations may have only negative or irrational solutions, while others may have no real solutions at all.

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