Question
Is the function defined by $\text{f(x)}= \begin{cases}\text{x} + 5,\ \ \text{if x}\leq 1 \\\text{x} - 5,\ \ \text{if x}>1\end{cases}$
a continuous function?

Answer

Here $\text{f(x)}= \begin{cases}\text{x} + 5,\ \ \text{if x}\leq 1 \\\text{x} - 5,\ \ \text{if x}>1\end{cases}$

Function f is defined at all points of the real line.

Let c be any real number.

Three cases arise:

Case I: c < 1

$^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{c}}\text{f(x)} = ^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{c}}\text{(x} + 5) = \text{c} + 5$

f(c) = c + 5

$\therefore\ ^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{c}}\text{f(x)} = \text{f(c)}$

$\therefore$ f is continuous at all points x < 1.

Case II: c > 1

$^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{c}}\text{f(x)} = ^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{c}}\text{f(x} - 5) = \text{c} - 5$

f(c) = c - 5

$\therefore\ ^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{c}}\text{f(x)} = \text{f(c)}$

$\therefore$ f is continuous at all points x > 1.

Case III: c = 1

$^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{1}^{-}}\text{f(x)} = ^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{1}^{-}}\text{(x} + 5) = 1 + 5 = 6$

$^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{1}^{+}}\text{f(x)} = ^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{1}^{+}}\text{(x} - 5) = 1 - 5 = -4$

$\therefore\ ^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{1}^{-}}\text{f(x)} \neq ^{\ \ \text{Lt}}_{\text{x}\rightarrow\text{1}^{+}}\text{f(x)}$

$\therefore$ f is discontinuous at x = 1.

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