Solve Calculus Question: Positive and Negative Cases for x > 1/3

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Discussion Overview

The discussion revolves around solving the inequality 1/x < 3, focusing on both positive and negative cases for x. Participants explore analytical methods for solving the inequality, particularly how to handle the sign changes when multiplying by negative numbers.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant states that for positive x, the solution is straightforward: x > 1/3.
  • Another participant explains that when x is negative, multiplying the inequality by x reverses the inequality, leading to the conclusion that x < 0 and x < 1/3.
  • A participant expresses confusion about the process of multiplying by negative numbers and whether the sign must be switched, seeking clarification on the algebraic steps involved.
  • Another participant emphasizes the importance of tracking whether a number is positive or negative when multiplying or dividing in inequalities, reiterating that this affects the direction of the inequality.
  • There is a challenge to the idea of simply accepting the rule about sign changes, with a participant questioning the understanding of this concept in a broader mathematical context.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to solve the inequality, and there is ongoing confusion regarding the rules for manipulating inequalities involving negative numbers. Multiple viewpoints on the process and understanding of the algebraic steps remain present.

Contextual Notes

Participants express uncertainty about the implications of multiplying by negative numbers and the necessity of switching signs, indicating potential gaps in foundational understanding. The discussion does not resolve these uncertainties.

shekki510
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we just started our calc class, and our teacher gave us this question
im sure it is really easy, and i don't know why i just can't get it analytically
the question is solve

1
- < 3 solve for positive and negative case.
x

the positive value is easy, X> 1/3 . How would you solve it for negative vaulues analytically?
 
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Multiplying an inequality by a positive number does not change the direction of the inequality. That's why you can say: For x positive, if 1/x< 3 then 1< 3x and 1/3< x.
(I first multiplied by x and then by 1/3, both positive numbers.)

Multiplying an inequality by a negative number reverses the direction of the inequality: For x negative, if 1/x< 3, then 1> 3x (multiply by the negative x and reverse the inequality. Now multiply by the positive number 1/3 and get 1/3> x.
BUT this was assuming x< 0. Since any x< 0 is necessarily less than 1/3, the solution set is: All x> 1/3 and all x< 0.
 
hmm that clears up some stuff i guess but I am not quite sure yet

so the problem is 1/x<3. multiply -1 to both sides, and you get -1/x > -3 (signs are switched) then to solve for X... -1>-3x then divide by -3 (signs are switched?) so x>1/3... i don't get how to get it algebraically. am i just supposed to accept that when u multiply by a negative number, u just swtich the signs and solve?
 
shekki510 said:
hmm that clears up some stuff i guess but I am not quite sure yet

so the problem is 1/x<3. multiply -1 to both sides, and you get -1/x > -3 (signs are switched) then to solve for X... -1>-3x then divide by -3 (signs are switched?) so x>1/3... i don't get how to get it algebraically. am i just supposed to accept that when u multiply by a negative number, u just swtich the signs and solve?

I think you're making it too complicated. Look at it like this:

Given: 1/x < 3, with x < 0.

Now, multiply both sides by x, remembering that when you multiply an inequality by a negative quantity, you change the sense of the inequality. So, you have:

1 > 3x

Now, divide by the positive 3 and you get: (you don't have to change the sense of the inequality now remember)

1/3 > x

You just need to keep track of whether you are multiplying/dividing by a negative or positive entity. Other than that, just solve like a regular algebraic equation.
 
shekki510 said:
hmm that clears up some stuff i guess but I am not quite sure yet

so the problem is 1/x<3. multiply -1 to both sides, and you get -1/x > -3 (signs are switched) then to solve for X... -1>-3x then divide by -3 (signs are switched?) so x>1/3... i don't get how to get it algebraically. am i just supposed to accept that when u multiply by a negative number, u just swtich the signs and solve?

No, I didn't say anything about multiplying by -1. I said multiply by x, in order to get it out of the denominator. To do that you have to consider whether x is positive or negative, just like your teacher told you.

And, no, you are not "just supposed to accept that when u multiply by a negative number, u just swtich the signs and solve?". You are supposed to have learned that when you learned arithmetic:

3< 5. How do -3 and -5 compare?
 

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