Trouble with needless of absolute value

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Discussion Overview

The discussion revolves around the use of absolute values in the context of solving differential equations, particularly when integrating functions. Participants explore scenarios where absolute values may or may not be necessary based on the physical meaning of the variables involved.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that while solving differential equations, absolute values are sometimes required, but in cases where a variable represents a quantity that cannot be negative (like atomic number), they may not be necessary.
  • Another participant agrees that the equation ln|y + 1| = ln|x| + constant leads to y + 1 = ax after removing logarithms, suggesting that absolute values are not needed in the final expression.
  • A different participant challenges this by stating that y + 1 could be positive while ax could be negative, implying that y + 1 could equal -ax, which would lead to different equalities.
  • In response, another participant argues that the constant "a" is determined by initial conditions, asserting that y + 1 and x will always have the same sign based on the initial values provided.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of absolute values in the equations discussed. While some argue that absolute values can be omitted in certain contexts, others contend that the signs of the variables could lead to different outcomes, indicating an unresolved debate.

Contextual Notes

The discussion highlights the dependence on initial conditions and the physical meanings of the variables involved, which may affect the necessity of absolute values in the equations.

mech-eng
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mary.png
Hi, while solving differential equations problems we have to sometimes use absolute value while
taking an integral of which 1 over a function. But in some problems I understand that we do not
have to use absolute value sign for example if the function related with atom number which can
never be a negative number we do not have to use absolute value. But in the picture I added here there is no sign about physical meaning of the problem so I think we should use absolute value and so there should have to be two different answers. The second picture of handwritting is
my other solution. The last one is a topic related with the problems. Have a nice day.
 

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Equation 2.8 certainly should be ln|y+ 1|= ln|x|+ cnst= ln|x|+ ln|a|= ln|ax|

However, once we have gotten rid of the logarithms on both sides, we have y+ 1= ax, whether y, x, and a are positive or negative, and no longer need the absolute value signs.
 
HallsofIvy said:
Equation 2.8 certainly should be ln|y+ 1|= ln|x|+ cnst= ln|x|+ ln|a|= ln|ax|

However, once we have gotten rid of the logarithms on both sides, we have y+ 1= ax, whether y, x, and a are positive or negative, and no longer need the absolute value signs.

But in this situation y +1 might be positive whereas ax might negative so y + 1 might equal to -ax which leads to two different equalities, isn't it?
 
Last edited:
No, that can't happen. "a" is a constant determined by the initial value. If we are given an initial value (x_0, y_0) such that y_0+ 1 and x_0 have the same sign, then a will be positive and y+ 1 and x will have the same sign for all x. If we are given an initial value (x_0, y_0) such that y_0+ 1 and x_0 have opposite sign, then a is negative and y+1 and x will have opposite sign for all x. In either case, y+ 1 and ax will have the same sign
 

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