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# 2.3E: Limit Laws & Techniques for Computing Limits EXERCISES

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## 2.3: The Limit Laws

In the following exercises, use the limit laws to evaluate each limit. Justify each step by indicating the appropriate limit law(s).

1) $$lim_{x→0}(4x^2−2x+3)$$

Solution: Use constant multiple law and difference law:

$$lim_{x→0}(4x^2−2x+3)=4lim_{x→0}x^2−2lim_{x→0}x+lim_{x→0}3=3$$

2) $$lim_{x→1}\frac{x^3+3x^2+5}{4−7x}$$

3) $$lim_{x→−2}\sqrt{x^2−6x+3}$$

Solution: Use root law: $$lim_{x→−2}\sqrt{x^2−6x+3}=\sqrt{lim_{x→−2}(x2−6x+3)}=\sqrt{19}$$

4) $$lim_{x→−1}(9x+1)^2$$

In the following exercises, use direct substitution to evaluate each limit.

5) $$lim_{x→7}x^2)$$

Solution: 49

6) $$lim_{x→−2}(4x^2−1)$$

7) $$lim_{x→0}\frac{1}{1+sinx}$$

Solution: 1

8) $$lim_{x→2}e^{2x−x^2}$$

9) $$lim_{x→1}\frac{2−7x}{x+6}$$

Solution: $$−\frac{5}{7}$$

10) $$lim_{x→3}lne^{3x}$$

In the following exercises, use direct substitution to show that each limit leads to the indeterminate form $$0/0$$. Then, evaluate the limit.

11) $$lim_{x→4}\frac{x^2−16}{x−4}$$

Solution:$$lim_{x→4}\frac{x^2−16}{x−4}=\frac{16−16}{4−4}=\frac{0}{0}; then, lim_{x→4}\frac{x^2−16}{x−4}= lim_{x→4}\frac{(x+4)(x−4)}{x−4}=8$$

12) $$lim_{x→2}\frac{x−2}{x^2−2x}$$

13) $$lim_{x→6}\frac{3x−18}{2x−12}$$

Solution: $$lim_{x→6}\frac{3x−18}{2x−12}=\frac{18−18}{12−12}=\frac{0}{0}; then, lim_{x→6}\frac{3x−18}{2x− 12}=lim_{x→6}\frac{3(x−6)}{2(x−6)}=\frac{3}{2}$$

14) $$lim_{h→0}\frac{(1+h)^2−1}{h}$$

15) $$lim_{t→9}\frac{t−9}{\sqrt{t−3}}$$

Solution: $$lim_{x→9}\frac{t−9}{\sqrt{t}−3}=\frac{9−9}{3−3}=\frac{0}{0}; then, lim_{t→9}\frac{t−9}{\sqrt{t}−3} =lim_{t→9}\frac{t−9}{\sqrt{t}−3}\frac{\sqrt{t}+3}{\sqrt{t}+3}=lim_{t→9}(\sqrt{t}+3)=6$$

16) $$lim_{h→0}\frac{\frac{1}{a+h}−\frac{1}{a}}{h}$$, where a is a real-valued constant

17) $$lim_{θ→π}\frac{sinθ}{tanθ}$$

Solution: $$lim_{θ→π}\frac{sinθ}{tanθ}=\frac{sinπ}{tanπ}=\frac{0}{0}; then, lim_{θ→π}\frac{sinθ}{tanθ}=lim_{θ→ π}\frac{sinθ}{\frac{sinθ}{cosθ}}=lim_{θ→π}cosθ=−1$$

18) $$lim_{x→1}\frac{x^3−1}{x^2−1}$$

19) $$lim_{x→1/2}\frac{2x^2+3x−2}{2x−1}$$

Solution: $$lim_{x→1/2}\frac{2x^2+3x−2}{2x−1}=\frac{\frac{1}{2}+\frac{3}{2}−2}{1−1}=\frac{0}{0}; then, lim_{x→ 1/2}\frac{2x^2+3x−2}{2x−1}=lim_{x→1/2}frac{(2x−1)(x+2)}{2x−1}=\frac{5}{2}$$

20) $$lim_{x→−3}\frac{\sqrt{x+4}−1}{x+3}$$

In the following exercises, use direct substitution to obtain an undefined expression. Then, use the method of Example to simplify the function to help determine the limit.

21) $$lim_{x→−2^−}\frac{2x^2+7x−4}{x^2+x−2}$$

Solution: −∞

22) $$lim_{x→−2^+}\frac{2x^2+7x−4}{x^2+x−2}$$

23) $$lim_{x→1^−}\frac{2x^2+7x−4}{x^2+x−2}$$

Solution: −∞

24) $$lim_{x→1^+}\frac{2x^2+7x−4}{x^2+x−2}$$

In the following exercises, assume that $$lim_{x→6}f(x)=4,lim_{x→6}g(x)=9$$, and $$lim_{x→6}h(x)=6$$. Use these three facts and the limit laws to evaluate each limit

25) $$lim_{x→6}2f(x)g(x)$$

Solution: $$lim_{x→6}2f(x)g(x)=2lim_{x→6}f(x)lim_{x→6}g(x)=72$$

26) $$lim_{x→6}\frac{g(x)−1}{f(x)}$$

27) $$lim_{x→6}(f(x)+\frac{1}{3}g(x))$$

Solution: $$lim_{x→6}(f(x)+\frac{1}{3}g(x))=lim_{x→6}f(x)+\frac{1}{3}lim_{x→6}g(x)=7$$\

28) $$lim_{x→6}\frac{(h(x))^3}{2}$$

29) $$lim_{x→6}\sqrt{g(x)−f(x)}$$

Solution: $$lim_{x→6}\sqrt{g(x)−f(x)}=\sqrt{lim_{x→6}g(x)−lim_{x→6}f(x)}=\sqrt{5}$$

30) $$lim_{x→6}x⋅h(x)$$

31) $$lim_{x→6}[(x+1)⋅f(x)]$$

Solution: $$lim_{x→6}[(x+1)f(x)]=(lim_{x→6}(x+1))(lim_{x→6}f(x))=28$$

32) $$lim_{x→6}(f(x)⋅g(x)−h(x))$$

[T] In the following exercises, use a calculator to draw the graph of each piecewise-defined function and study the graph to evaluate the given limits.

33) $$f(x)=\begin{cases}x2 & x≤3,\\ x+4 & x>3\end{cases}$$

1. a. $$lim_{x→3^−}f(x)$$
2. b. $$lim_{x→3^+}f(x)$$

Solution:

a. 9; b. 7

34) $$g(x)=\begin{cases}x^3−1 & x≤0\\1 & x>0\end{cases}$$

1. a. $$lim_{x→0^−}g(x)$$
2. b. $$lim_{x→0^+}g(x)$$

35) $$h(x)=\begin{cases}x^2−2x+1 & x<2x≥2\\3−x & x≥2\end{cases}$$

1. a. $$lim_{x→2^−}h(x)$$
2. b. $$lim_{x→2^+}h(x)$$

In the following exercises, use the following graphs and the limit laws to evaluate each limit.

36) $$lim_{x→−3^+}(f(x)+g(x))$$

37) $$lim_{x→−3^−}(f(x)−3g(x))$$

Solution: $$lim_{x→−3^−}(f(x)−3g(x))=lim_{x→−3^−}f(x)−3lim_{x→−3^−}g(x)=0+6=6$$

38) $$lim_{x→0}\frac{f(x)g(x)}{3}$$

39) $$lim_{x→−5}\frac{2+g(x)}{f(x)}$$

Solution: $$lim_{x→−5}\frac{2+g(x)}{f(x)}=\frac{2+(lim_{x→−5}g(x))}{lim_{x→−5}f(x)}=\frac{2+0}{2}=1$$

40) $$lim_{x→1}(f(x))^2$$

41) $$lim_{x→1}\sqrt{f(x)−g(x)}$$

Solution: $$lim_{x→1}\sqrt[3]{f(x)−g(x)}=\sqrt[3]{lim_{x→1}f(x)−lim_{x→1}g(x)}=\sqrt[3]{2+5}=\sqrt[3]{7}$$

42) $$lim_{x→−7}(x⋅g(x))$$

43) $$lim_{x→−9}[x⋅f(x)+2⋅g(x)]$$

Solution: $$lim_{x→−9}(xf(x)+2g(x))=(lim_{x→−9}x)(lim_{x→−9}f(x))+2lim_{x→−9}(g(x))=(−9)(6)+2(4)=−46$$

For the following problems, evaluate the limit using the squeeze theorem. Use a calculator to graph the functions $$f(x),g(x)$$, and $$h(x)$$ when possible.

44) [T] True or False? If $$2x−1≤g(x)≤x^2−2x+3$$, then $$lim_{x→2}g(x)=0$$.

45) [T] $$lim_{θ→0}θ^2cos(\frac{1}{θ}) Solution: The limit is zero. 46) \(lim_{x→0}f(x)$$, where $$f(x)=\begin{cases}0 & x rational\\ x^2 & x irrrational\end{cases}$$

47) [T] In physics, the magnitude of an electric field generated by a point charge at a distance r in vacuum is governed by Coulomb’s law: $$E(r)=\frac{q}{4πε0_r^2}$$, where E represents the magnitude of the electric field, q is the charge of the particle, r is the distance between the particle and where the strength of the field is measured, and \frac{1}{4πε_0} is Coulomb’s constant: $$8.988×109N⋅m^2/C^2$$.

a. Use a graphing calculator to graph $$E(r)$$ given that the charge of the particle is $$q=10^{−10}$$.

b. Evaluate $$lim_{r→0^+}E(r)$$. What is the physical meaning of this quantity? Is it physically relevant? Why are you evaluating from the right?

Solution: a

b. ∞. The magnitude of the electric field as you approach the particle q becomes infinite. It does not make physical sense to evaluate negative distance.

48) [T] The density of an object is given by its mass divided by its volume: $$ρ=m/V.$$

a. Use a calculator to plot the volume as a function of density $$(V=m/ρ)$$, assuming you are examining something of mass 8 kg ($$m=8$$).

b. Evaluate $$lim_{x→0^+}V(\rho)$$ and explain the physical meaning.