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Question

Decide whether the function is continuous on the given interval. If not, define or redefine the function at one point to make it continuous everywhere on the given interval, or else explain why that cannot be done.

f(x)=x+4x8 on [0,10]f(x)=\frac{x+4}{x-8} \text { on }[0,10]

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The rational function f(x)=x+4x8f ( x ) = \frac { x + 4 } { x - 8 } is conntinuous everywhen x80x-8\neq 0. In other words, f(x)f(x) is continuous everywhere except at x=8x=8. The limit at this point does not exist because

limx8f(x)=limx8x+4x8=   x<8x8<0 the argument decreases without bound when x approaches 8 from the left\begin{aligned} \lim _ { x \rightarrow 8 ^ { - } } f ( x ) = \lim _ { x \rightarrow 8 ^ { - } } \frac { x + 4 } { x - 8 }=-\infty\ \ \ x<8\Rightarrow x-8<0\\ \Rightarrow \ \text{the argument decreases without bound when x approaches 8 from the left} \end{aligned}

and

limx8+f(x)=limx8+x+4x8=+   because x>8  and the argument increases without bound\begin{aligned} \lim _ { x \rightarrow 8 ^ { + } } f ( x ) = \lim _ { x \rightarrow 8 ^+} \frac { x + 4 } { x - 8 } = +\infty \ \ \ \text{because}\ x>8\ \text{ and the argument increases without bound} \end{aligned}

Since the limit at x=8[0,10]x = 8 \in [ 0,10 ] does not exist, f(x)f(x) is discontinuous on the prescribed interval

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