Proving Continuity in Functions: A Comparison of Two Statements

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In summary, if f is a discontinuous function on the real line and sin(f(x)) is continuous at some point on the real line, then f is continuous as well.
  • #1
stukbv
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Homework Statement



1. if f : [-1,1] --> Reals is such that sin(f(x) is continuous on the reals then f is continuous.

2. if f : [-1,1] --> Reals is such that f(sin(x)) is continuous on the reals then f is continuous.

Are these true or false how do i prove / give a counter example?
 
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  • #2


Again "If you do not show at least some attempt to do this problem yourself, this thread will be deleted."
 
  • #3


ok, its just i have no idea where to start really.
I know that the composition of 2 continuous functions is continuous too so I think that the second one is true, since sinx is continuous everywhere, the only way the composition could be discontinuous is is f was discontinuous??
 
  • #4


Hi stukbv! :smile:

Take your favorite discontinuous function and try it out!
 
  • #5


are you talking about number 1 or 2? I think i have disproved number 1 so that's okay.. now just 2 :(
 
  • #6


stukbv said:
are you talking about number 1 or 2? I think i have disproved number 1 so that's okay.. now just 2 :(

Try to prove that one. What definition of continuity would you like to use?
 
  • #7


e-d i think ,

so for |x-c| < d we have that |f(sin(x))-f(sin(c))| < e
 
  • #8


So for our epsilon we need to find a delta such that

[tex]|x-c|<\delta~\Rightarrow~|f(x)-f(c)|<\varepsilon[/tex]

Now, the trick is, can you write that x and c as sines of something close together?
 
  • #9


i don't know what you mean
 
  • #10


Can you write x=sin(y) and c=sin(d)?
 
  • #11


oh ok so we get |sin(y)-sin(d)| < delta => |f(sin(y))-f(sin(d))| < epsilon?
 
  • #12


stukbv said:
oh ok so we get |sin(y)-sin(d)| < delta => |f(sin(y))-f(sin(d))| < epsilon?

Yes, and if you know that |y-d| is small, then |f(sin(y))-f(sin(d))| is also small, by hypothesis...
 
  • #13


so is that it then ?
 
  • #14


Once you've checked that it is indeed possible to take y and d close together, then that's that! :smile:
 
  • #15


arghh I am confused how would i check - sorry to be such a pain~!
 
  • #16


Use the continuity of the inverse sine function...
 

FAQ: Proving Continuity in Functions: A Comparison of Two Statements

What is a continuity math problem?

A continuity math problem is a type of math problem that involves finding the value of a function at a specific point by analyzing its behavior around that point. It is based on the concept of continuity, which states that a function is continuous if there are no breaks or jumps in its graph.

How do you determine if a function is continuous?

A function is continuous if its graph is a single, unbroken curve with no gaps or holes. This means that the function must exist and have a defined value at every point along the x-axis. Additionally, the left and right limits of the function at a specific point must be equal.

What is the role of limits in solving continuity math problems?

Limits are used to determine the behavior of a function around a specific point. In continuity math problems, limits help to determine if a function is continuous at a given point by comparing the value of the function at that point to the values of the function approaching that point from both the left and right sides.

How do you solve a continuity math problem?

To solve a continuity math problem, you must first determine if the function is continuous at the given point. If it is, then the value of the function at that point is equal to the limit of the function as x approaches that point. If the function is not continuous, you must use algebraic techniques to find the correct value of the function at the given point.

What are some common types of continuity math problems?

Some common types of continuity math problems include finding the value of a piecewise function at a specific point, determining if a function is continuous on a closed interval, and solving for the value of a function at a point using the limit definition of continuity. Other types may involve logarithmic or exponential functions, trigonometric functions, or polynomial functions.

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