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EXERCISE 8.1 · Q28

Q.Which of the following functions has a removable discontinuity? If it has a removable discontinuity, redefine the function so that it becomes continuous: f(x)=x3−8x2−4f(x) = \dfrac{x^3-8}{x^2-4}, for x>2x > 2, =3= 3, for x=2x = 2, =e3(x−2)2−12(x−2)2= \dfrac{e^{3(x-2)^2}-1}{2(x-2)^2}, for x<2x < 2.

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f(x)=x3−8x2−4f(x)=\dfrac{x^3-8}{x^2-4} for x>2x>2, f(2)=3f(2)=3, and f(x)=e3(x−2)2−12(x−2)2f(x)=\dfrac{e^{3(x-2)^2}-1}{2(x-2)^2} for x<2x<2.

Right-hand limit: factor x3−8=(x−2)(x2+2x+4)x^3-8=(x-2)(x^2+2x+4) and x2−4=(x−2)(x+2)x^2-4=(x-2)(x+2), so for x≠2x\ne2, x3−8x2−4=x2+2x+4x+2\dfrac{x^3-8}{x^2-4}=\dfrac{x^2+2x+4}{x+2}, giving lim⁡x→2+f(x)=4+4+44=124=3\displaystyle\lim_{x\to2^+} f(x)=\dfrac{4+4+4}{4}=\dfrac{12}{4}=3.

Left-hand limit: put h=(x−2)2→0+h=(x-2)^2\to0^+ as x→2−x\to2^-; then lim⁡x→2−f(x)=lim⁡h→0+e3h−12h=12lim⁡h→0e3h−1h=12(3)=32\displaystyle\lim_{x\to2^-} f(x)=\lim_{h\to0^+}\frac{e^{3h}-1}{2h}=\frac12\lim_{h\to0}\frac{e^{3h}-1}{h}=\frac12(3)=\frac32, using lim⁡h→0(e3h−1)/h=3\lim_{h\to0}(e^{3h}-1)/h=3. …

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