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Continuity of Functions

Continuity of Functions

Functions and Combinations of Functions

Many functions are continuous at every real number x. These functions include (but are not limited to):

  1. all polynomials (including lines)
  2.  ex
  3. sin(x) and cos(x)

It's helpful to see the continuity by graphing the functions. If we graph any of the above functions, we see a nice smooth graph that continues across the whole x-axis, with no jumps or holes. Try it, it will bring you and your TI 83 closer together.

Many other functions are continuous everywhere that they're defined, including

  1. ln(x) - This is continuous for all > 0
  2. tan(x) - This is continuous in between multiples of  and  
  3. all rational functions that don't have common roots in the numerator and denominator - These will have vertical asymptotes at the roots of the denominator, and be continuous in between those asymptotes.

Once we know a couple of functions that are continuous at a point c, we can build other functions that are continuous at c by combining the functions we already have. To do this, we use some properties of limits.

If f and g are continuous at c, then

1. We can add or subtract:

(f + g) and (g) are continuous at c

2. We can multiply:

(fg) is continuous at c

3. We can divide functions:

 is continuous at c as long as g(c) ≠ 0

4. We can compose:

The composition (f ο g) is continuous at c.

All that's required here is that we have two functions continuous at c. It doesn't matter which is f and which is g. By switching f and g in our minds, we also find that (g - f) is continuous at c, g ο f is continuous at c, etc.

Sample Problem

Let f(x) = x + 1 and g(x) = ex. These functions are both continuous at every real number x. The following functions are also continuous at every real number x:

1. We can add or subtract:

(f + g)(x) = x + 1 + ex (which is the same as (g + f)(x))

(f - g)(x) = (x + 1)- ex

(g - f)(x) = e- (x + 1) = e- 1

2. We can multiply:

(fg)(x) = (x + 1)(ex) = xex + ex (which is the same as (gf)(x))

3. We can divide:

 (this is continuous at every real number since ex is never 0)

4. We can compose:

(f ο g)(x) = (ex) + 1

(g ο f)(x) = ex + 1

Also, the function is continuous at every real number except x = -1.

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