Calculating limits by subbing in values

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Discussion Overview

The discussion revolves around the concept of calculating limits in mathematics, specifically addressing the method of substituting values into functions to determine limits. Participants explore the implications of this method, particularly in relation to continuity and the behavior of functions at specific points.

Discussion Character

  • Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions whether substituting values to solve limits undermines the purpose of limits, particularly in cases where the limit as x approaches a may not equal f(a).
  • Another participant attempts to clarify the concept by explaining that limits focus on the y-value a function approaches, regardless of any discontinuities at that point.
  • A participant notes that for some limits, direct substitution is valid if the denominator is not equal to zero, and mentions the concept of continuity when the limit equals the function value at that point.
  • A later reply indicates that the original question was resolved in a lecture, referencing the direct substitution property as a formal concept.

Areas of Agreement / Disagreement

Participants express differing views on the validity and implications of using substitution to calculate limits. The discussion does not reach a consensus, as some participants support the method while others raise concerns about its limitations.

Contextual Notes

Limitations include the potential misunderstanding of continuity and the conditions under which direct substitution is applicable. The discussion also touches on the existence of discontinuities and their impact on limit calculations.

tahayassen
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Doesn't solving limits by substituting values defeat the point? For example: you can solve a limit of a quadratic by just subbing in a x value. But how do we know limit as x approaches a is the same as f(a)?
 
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Please clarify with an example.
 
I'll try to take a stab at what you're getting at :)

For limits, we are concerned what y-value the function is headed for. It doesn't matter if there is a hole or something at that point, because the limit never actually equals the value x is headed towards. It can get infinitely close, however.

For some limits, we don't have to do much work or simplification. So, if you can plug in the x-value without the denominator being equal to 0, go for it!

Also, if lim x-> a of f(x) exits and equals f(a) , we call the function continuous (Assuming f(a) exists as well)
 
Never mind.

My prof. answered my question in lecture today. It's called the direct substitution property.

http://web.viu.ca/wattsv/math121/Overheads/section14/directsubstitution.pdf
 
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