# 6.E: Continuity - What It Isn’t and What It Is (Exercises)

- Page ID
- 7955

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### Q1

Use the deﬁnition of continuity to prove that the constant function \(g(x) = c\) is continuous at any point a.

### Q2

- Use the deﬁnition of continuity to prove that \(\ln x\) is continuous at \(1\). [
You may want to use the fact \(\left |\ln x \right | < \varepsilon \Leftrightarrow -\varepsilon < \ln x < \varepsilon\) to ﬁnd a \(δ\).]**Hint:** - Use part (a) to prove that \(\ln x\) is continuous at any positive real number \(a\). [
\(\ln (x) = \ln (x/a) + \ln (a)\). This is a combination of functions which are continuous at \(a\). Be sure to explain how you know that \(\ln (x/a)\) is continuous at \(a\).]*Hint:*

### Q3

Write a formal deﬁnition of the statement \(f\) is not continuous at \(a\), and use it to prove that the function \(f(x) = \begin{cases} x & \text{ if } x\neq 1 \\ 0 & \text{ if } x= 1 \end{cases}\) is not continuous at \(a = 1\).

### Contributor

Eugene Boman (Pennsylvania State University) and Robert Rogers (SUNY Fredonia)