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Finding value of c to make left/right continuous

  1. Sep 17, 2013 #1
    1. The problem statement, all variables and given/known data
    Given:
    g(x) = {x+3 for x<-1, cx for -1≤x≤2, x+2 for x>2

    Questions:
    Find value of c such that g(x) is a)left continuous. b)right continuous.


    2. Relevant equations
    ??


    3. The attempt at a solution
    I tried using a method that used finding a point such as (-1,x) and (2,x) but seeing how c is in the middle equation, that didn't work out so well. So I'm stuck as to what to do.
    I've done a question sort of like this a long time ago but it was just asking for f(x) to be continuous. Didn't specifically ask about left/right so I'm stumped there.
     
  2. jcsd
  3. Sep 17, 2013 #2

    mfb

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    There is one value of x where "left continuous" can be wrong, and one point where "right continuous" can be wrong. Can you find those two points?
    For (a) you have to care about the first one, for (b) the second one is relevant.
     
  4. Sep 17, 2013 #3
    Actually no I don't know how to find those two points sadly.. And what do you mean about you have to care about the first one, and for b the second one is relevant
     
  5. Sep 17, 2013 #4

    mfb

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    Is the function left-continuous at x=-4?
    Is the function left-continuous at x=0?

    Is the function left-continuous within those 3 regions of the function definition?
    Is the function left-continuous at the first border between those regions?
    Is the function left-continuous at the second border between those regions?
    Is there any x-value not covered with these three questions?

    If one of those answers depend on c, which value of c do you need?
     
  6. Sep 17, 2013 #5
    Since x+3 is valid at x=-4, isn't it left continuous?
    How do I check for continuity at x=0? Since it says the function is at cx

    I guess the main question for me is how am I supposed to find out how to check if its continuous from a side.
     
  7. Sep 17, 2013 #6

    mfb

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    "valid"?
    At that point, it is continuous, which implies left continuous.

    With the definition of continuity.

    As with from both sides, you can just ignore function values on one side (so it is easier in some cases).
     
  8. Sep 17, 2013 #7
    Okay, so if the function is continuous until x<-1,to check for this left continuity do I plug -1 into the first equation, giving 2. And then do I plug in -1 into the second equation giving -c? But 2 =/= -c.

    That's all I'm getting from this, sorry I don't understand it very well..
     
  9. Sep 17, 2013 #8

    mfb

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    Why? What prevents -c to be 2?
     
  10. Sep 17, 2013 #9
    Well I just thought that if 2 is the answer to the first, and -c is the answer to the second, then since they aren't the exact same values, how can they be continuous
     
  11. Sep 17, 2013 #10

    CAF123

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    Sometimes it helps to actually sketch the function. Sketch y=x+3 for x<-1 and y=x+2 for x>2. You can leave the interval -1<x<2 clear for just now, but if you drew the sketch accurately, it should quickly help you determine the sign of c.
     
  12. Sep 17, 2013 #11
    yes I see the sign of c is positive. How can I just find the value of c from this? Is it just 1?
     
  13. Sep 17, 2013 #12

    CAF123

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    Yes Edit: misread question, sign of c is clear depending on whether you consider the left continuity or right continuity.
    It is not what I get. Since each function in each interval are linear functions or polynomials of degree 1, they are continuous on the real line. In particular, ##\lim_{x \rightarrow a} f(x) = f(a)## Use your sketch to decide what 'a' to use and what function to use in each interval.
     
    Last edited: Sep 17, 2013
  14. Sep 17, 2013 #13
    So I used a=-1 for the x+3 interval, which makes 2. I just don't understand where I'm supposed to go from this step. That "simple" limit you showed is odd to me, the f(a) doesn't make sense to me I guess.
     
  15. Sep 17, 2013 #14
    If the limit of the first interval is 2, wouldn't this mean that the c value would have to be 2?
     
  16. Sep 17, 2013 #15

    CAF123

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    Apologies, I completely misread the question at first. Ignore my previous comment about the sign of c. What you have found above is that ##\lim_{x \rightarrow -1^-} x+3 = 2##. What you need to find in a) is the value of c that makes g(x) right continuous. This means as you approach -1 from the right, the value of cx (we use cx here since g(x) = cx for -1<x<2) tends to 2.
     
  17. Sep 17, 2013 #16
    Alright so I clarified that 2 is the answer for a). That part makes sense now. But for b), how would the answer be any different? Since the lines will be in the same spots either way, would it be -2?
     
  18. Sep 17, 2013 #17

    CAF123

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    Yes, thats correct, the lines both go through the origin if that is what you mean. I am not sure how you made your deduction though. For b), you want to find the value of c that makes ##\lim_{x \rightarrow -1^+} cx = \lim_{x \rightarrow -1^-} x + 3= 2##
     
  19. Sep 17, 2013 #18
    What do you mean by thats correct, which part? The answer being any different or the -2? So b) is either 2 or -2?
     
  20. Sep 17, 2013 #19

    CAF123

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    I mean that both answers are correct, that is c is -2 for (b) and 2 for (a), but I didn't quite understand your reasoning for (b).
     
  21. Sep 17, 2013 #20
    Oh well I just assumed that since it's left continuous at 2 going in a positive direction, then if it's going in a negative direction(to the right) then it would have to be -2.
     
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