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Strategy in finding limits

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Recall

Definition. A function f is a rule, relationship, or correspondence that assigns to each element of one set (x ∈ D), called the domain, exactly one element of a second set, called the range (y ∈ E).

The pair (x, y) is denoted as y = f(x). Typically, the sets D and E will be both the set of real numbers, ℝ. A mathematical function is like a black box that takes certain input values and generates corresponding output values (Figure E).

 

Very loosing speaking, a limit is the value to which a function grows close as the input get closer and closer to some other given value.

One would say that the limit of f, as x approaches a, is L, $\lim_{x \to a} f(x)=L$. Formally, for every real ε > 0, there exists a real δ > 0 such that for all real x, 0 < | x − a | < δ implies that | f(x) − L | < ε. In other words, f(x) gets closer and closer to L, f(x)∈ (L-ε, L+ε), as x moves closer and closer -approaching closer but never touching- to a (x ∈ (a-δ, a+δ), x≠a)) -Fig 1.a.-

  Definition. Let f(x) be a function defined on an interval that contains x = a, except possibly at x = a, then we say that, $\lim_{x \to a} f(x) = L$ if

$\forall \epsilon>0, \exists \delta>0: 0<|x-a|<\delta, implies~ |f(x)-L|<\epsilon$

Or

$\forall \epsilon>0, \exists \delta>0: |f(x)-L|<\epsilon, whenever~ 0<|x-a|<\delta$    

Strategy in finding limits

Don’t worry, you are not alone. Calculating limits can be confusing and challenging for many students. But don’t worry, you are in the right place. Here are some common strategies and techniques for evaluating limits:

  1. $\lim_{x \to 0} x^2 = 0$.
  2. $\lim_{x \to 2} 5x -4 = 5·2 -4 = 6$.
  1. $\lim_{x \to 2}\frac{x^2-4}{x-2} =\lim_{x \to 2}\frac{(x+2)(x-2)}{x-2} = \lim_{x \to 2}(x+2)=4$.
  2. $\lim_{x \to 4} \frac{\sqrt{x}-2}{x-4} = \lim_{x \to 4} \frac{(\sqrt{x}-2)(\sqrt{x}+2)}{(x-4)(\sqrt{x}+2)} = \lim_{x \to 4} \frac{x-4}{(x-4)(\sqrt{x}+2)} = \lim_{x \to 4} \frac{1}{\sqrt{x}+2} = \frac{1}{4}.$
  1. $\lim_{x \to 2} \frac{\frac{1}{x}-\frac{1}{2}}{x-2} = \lim_{x \to 2} \frac{\frac{2-x}{2x}}{x-2} = \lim_{x \to 2} \frac{-1}{2x} = \frac{-1}{4}$
  2. $\lim_{x \to 2}(\frac{1}{4x-8}-\frac{1}{x^2-4}) = \lim_{x \to 2} (\frac{1}{4(x-2)}-\frac{1}{(x-2)(x+2)}) = \lim_{x \to 2} (\frac{(x+2) - 4}{4(x-2)(x+2)}) = \lim_{x \to 2} \frac{x-2}{4(x-2)(x+2)} = \lim_{x \to 2} \frac{1}{4(x+2)} = \frac{1}{16}.$
  3. $\c$=[∞-∞] $\lim_{x \to 2} \frac{4-(x+2)}{(x-2)(x+2)} = \lim_{x \to 2} \frac{(2-x)}{(x-2)(x+2)} = \lim_{x \to 2} \frac{-1}{(x+2)} = \frac{-1}{4}.$
  1. $\lim_{x \to 0} \frac{\sqrt{1+x}-1}{x} = \lim_{x \to 0} \frac{\sqrt{1+x}-1}{x}·\frac{\sqrt{1+x}+1}{\sqrt{1+x}+1} = \lim_{x \to 0} \frac{(1+x)-1}{x(\sqrt{1+x}+1)} = \lim_{x \to 0} \frac{x}{x(\sqrt{1+x}+1)} = \lim_{x \to 0} \frac{1}{(\sqrt{1+x}+1)} = \frac{1}{2}$.
  2. $\lim_{x \to 0} (\frac{3}{x\sqrt{9-x}}-\frac{1}{x})$[Combine the fractions] $\lim_{x \to 0} (\frac{3-\sqrt{9-x}}{x\sqrt{9-x}}) = \lim_{x \to 0} (\frac{3-\sqrt{9-x}}{x\sqrt{9-x}}·\frac{3+\sqrt{9-x}}{3+\sqrt{9-x}}) = \lim_{x \to 0} \frac{9-(9-x)}{(x\sqrt{9-x})(3+\sqrt{9-x})} = \lim_{x \to 0} \frac{x}{(x\sqrt{9-x})(3+\sqrt{9-x})} = \lim_{x \to 0} \frac{1}{(\sqrt{9-x})(3+\sqrt{9-x})} = \frac{1}{\sqrt{9}(3+\sqrt{9})} = \frac{1}{3·6} = \frac{1}{18}$
  3. $\lim_{x \to 2} \frac{3-\sqrt{2x+5}}{x-2} = \lim_{x \to 2} \frac{3-\sqrt{2x+5}}{x-2}\frac{3+\sqrt{2x+5}}{3+\sqrt{2x+5}} = \lim_{x \to 2} \frac{9-(2x+5)}{(x-2)(3+\sqrt{2x+5})} = \lim_{x \to 2} \frac{-2x+4}{(x-2)(3+\sqrt{2x+5})} = \lim_{x \to 2} \frac{-2(x-2)}{(x-2)(3+\sqrt{2x+5})} = \lim_{x \to 2} \frac{-2}{(3+\sqrt{2x+5})} = \frac{-2}{3+\sqrt{9}} = \frac{-2}{6} = \frac{-1}{3}$
  4. $\lim_{x \to 9} \frac{x-9}{\sqrt{x}-3} = \lim_{x \to 9} \frac{(x-9)(\sqrt{x}+3)}{(\sqrt{x}-3)(\sqrt{x}+3)} = \lim_{x \to 9} \frac{(x-9)(\sqrt{x}+3)}{x-9} = \lim_{x \to 9} \sqrt{x}+3 = \sqrt{9} + 3 = 6.$
  5. $\lim_{x \to 0} \frac{1}{x\sqrt{x+1}}-\frac{1}{x} =$ [∞-∞] $\lim_{x \to 0} \frac{1-\sqrt{x+1}}{x\sqrt{x+1}} = \lim_{x \to 0} \frac{1-\sqrt{x+1}}{x\sqrt{x+1}} \frac{1+\sqrt{x+1}}{1+\sqrt{x+1}} = \lim_{x \to 0} \frac{1-(x+1)}{x\sqrt{x+1}(1+\sqrt{x+1})} = \lim_{x \to 0} \frac{-x}{x\sqrt{x+1}(1+\sqrt{x+1})} = \lim_{x \to 0} \frac{-1}{\sqrt{x+1}(1+\sqrt{x+1})} = \frac{-1}{2}$
  1. $\lim_{x \to 0} f(x), ~where~f(x)= x·sin(\frac{1}{x}) = 0$ for x ≠ 0, and f(0) = 0 (Figure i). $-1 ≤ sin(\frac{1}{x}) ≤ 1 ⇨ -x ≤ xsin(\frac{1}{x}) ≤ x$

    $\lim_{x \to 0} x = \lim_{x \to 0} -x = 0.$ Then, by the Squeeze Theorem, $\lim_{x \to 0} x·sin(\frac{1}{x}) = 0$.

  2. $\lim_{x \to 0} (\frac{sin(x)}{x}) = 1,$ (Figure iii)  

    $Area(\triangle ADF) ≥ Area(Sector ADB) ≥ Area(\triangle ADB)$ ⇨ $\frac{tan(x)}{2} ≥ \frac{x}{2} ≥ \frac{sin(x)}{2}$ because the Area of a circle with an angle measuring 360º is πr2 ⇨ Area of a sector (θ is measured in degrees) is θ360ºπr2 ⇨ Area(Sector ADB) = 12r2θ, where θ is the angle in radians =[r = 1] 12θ, and then consider that x = θ.

    $ ⇨ \frac{sin(x)}{cos(x)} ≥ x ≥ sin(x) ⇨ \frac{cos(x)}{sin(x)} ≤ \frac{1}{x} ≤\frac{1}{sin(x)} ⇨ cos(x) ≤ \frac{sin(x)}{x} ≤1$

    $\lim_{x \to 0} cos(x) = \lim_{x \to 0} 1 = 1$ ⇨[by the Squeeze Theorem] $\lim_{x \to 0} (\frac{sin(x)}{x}) = 1.$

  3. $\lim_{x \to 0} (\frac{1-cos(x)}{x}) = 0,$ (Figure iv). 

      $\lim_{x \to 0} cos(\frac{\pi-x}{2}) = \lim_{x \to 0} 0 = 0, \lim_{x \to 0} 0 = 0$ ⇨[By the squeeze theorem] $\lim_{x \to 0} (\frac{1-cos(x)}{x}) = 0.$

  4. $\lim_{x \to 0} x^2·e^{sin(\frac{1}{x})}$

    $-1 ≤ sin(\frac{1}{x}) ≤ 1 ⇒$[The exponential is a monotone increasing function] $e^{-1} ≤ e^{sin(\frac{1}{x})} ≤ e^1 ⇒ x^2e^{-1} ≤ x^2e^{sin(\frac{1}{x})} ≤ x^2e$

    $\lim_{x \to 0} x^2e = e·\lim_{x \to 0} x^2 = e·0 = 0, \lim_{x \to 0} x^2e^{-1} = e^{-1}·\lim_{x \to 0} x^2 = e^{-1}·0 = 0$ ⇒[By the Squeeze Theorem] $\lim_{x \to 0} x^2·e^{sin(\frac{1}{x})} = 0.$

  1. $\lim_{x \to 0} \frac{1}{x^2} = ∞$ because $\lim_{x \to 0⁺} \frac{1}{x^2} = \lim_{x \to 0⁻} \frac{1}{x^2} = ∞$.
  2. $\lim_{x \to 2} \frac{x^2+4}{x^3-8}$ does not exist because $\lim_{x \to 2⁺} \frac{x^2+4}{x^3-8} = \frac{8}{0⁺} = ∞, \lim_{x \to 2⁻} \frac{x^2+4}{x^3-8} = \frac{8}{0⁻} = -∞$.
  3. $\lim_{x \to 3} \frac{x-4}{x^2-6x+9} = \lim_{x \to 3} \frac{x-4}{(x-3)^2} = -∞$ because $\lim_{x \to 3⁺} \frac{x-4}{x^2-6x+9} = \lim_{x \to 3⁺} \frac{x-4}{(x-3)^2} = -∞ = \lim_{x \to 3⁻} \frac{x-4}{x^2-6x+9} = \lim_{x \to 3⁻} \frac{x-4}{(x-3)^2} = -∞$.
  1. $\lim_{θ \to 0} \frac{sin(θ)}{sin(2θ)}$ =[We can use the trigonometric identity sin(2θ) = 2sin(θ)cos(θ)] $\lim_{θ \to 0} \frac{sin(θ)}{2sin(θ)cos(θ)} = \lim_{θ \to 0} \frac{1}{2·cos(θ)} = \frac{1}{2·1} = \frac{1}{2}.$
  2. $\lim_{x \to \frac{π}{4}} \frac{sin(x)-cos(x)}{cos^2(x)-sin^2(x)}$ =[The denominator is a difference of perfect squares, let’s factor it] $\lim_{x \to \frac{π}{4}} \frac{sin(x)-cos(x)}{(cos(x)-sin(x))(cos(x)+sin(x))} = \lim_{x \to \frac{π}{4}} \frac{-1}{(cos(x)+sin(x))} = \frac{-1}{\frac{\sqrt{2}}{2}+\frac{\sqrt{2}}{2}} = \frac{-1}{\sqrt{2}} = \frac{-1}{\sqrt{2}}\frac{\sqrt{2}}{\sqrt{2}} = \frac{-\sqrt{2}}{2}$
  3. $\lim_{x \to \frac{π}{2}} \frac{cos(2x)}{tan(x)}$ =[cos(2θ) = cos2(θ)-sin2(θ) = 1 -2sin2(θ)] $\lim_{x \to \frac{π}{2}} \frac{1-2sin^2(x)}{\frac{sin(x)}{cos(x)}} = \lim_{x \to \frac{π}{2}} \frac{1-2sin^2(x)}{1}·\frac{cos(x)}{sin(x)} = \lim_{x \to \frac{π}{2}} \frac{(1-2sin^2(x))cos(x)}{sin(x)}=\frac{(1-2)·0}{1} = 0.$
  4. $\lim_{x \to 0} \frac{1-cos(x)}{x^2}$ = [$1-cos(x)=2sin^2(\frac{x}{2})$] = $\lim_{x \to 0} \frac{2sin^2(\frac{x}{2})}{x^2} =$[Multiplication law, states that the limit of a product of two functions equals the product of the limits of both functions, $\lim_{x \to a} (f(x)·g(x)) = lim_{x \to a} f(x) · lim_{x \to a} g(x).$ ] $2·\lim_{x \to 0} \frac{sin(\frac{x}{2})}{x}\lim_{x \to 0} \frac{sin(\frac{x}{2})}{x} = \frac{2}{4}·\lim_{x \to 0} \frac{sin(\frac{x}{2})}{\frac{x}{2}}\lim_{x \to 0} \frac{sin(\frac{x}{2})}{\frac{x}{2}} = \frac{2}{4}·1·1 = \frac{1}{2}$ Notice: Substitution is a general method used to simplify complicated expressions by assigning a simple name to some part of that expression, in our case $\frac{x}{2} = u$, and notice that x→0 ↭ u→0, and $\lim_{u \to 0} \frac{sin(u)}{u} = 1$ -The squeezing theorem, example 2-.
  5. $\lim_{x \to 0} \frac{1-cos(x)}{x^2} = \lim_{x \to 0} \frac{1-cos(x)}{x^2} \frac{1+cos(x)}{1+cos(x)} = \lim_{x \to 0} \frac{1-cos^2(x)}{x^2(1+cos(x))} =$ [cos2(x)+sin2(x) = 1 ⇒ 1 -cos2(x) = sin2(x)] $\lim_{x \to 0} \frac{sin^2(x)}{x^2(1+cos(x))}$ = [Multiplication law, states that the limit of a product of two functions equals the product of the limits of both functions, $\lim_{x \to a} (f(x)·g(x)) = lim_{x \to a} f(x) · lim_{x \to a} g(x).$ ] $\lim_{x \to 0} \frac{sin(x)}{x}·\lim_{x \to 0} \frac{sin(x)}{x}·\lim_{x \to 0} \frac{1}{1+cos(x)} = 1·1·\frac{1}{1+1} = \frac{1}{2}.$ Notice that $\lim_{u \to 0} \frac{sin(u)}{u} = 1$ -The squeezing theorem, example 2-.
  6. $\lim_{x \to 0} \frac{cos(2x)-1}{cos(x)-1}$ =[cos(a + b) = cos(a)·cos(b) - sin(a)·sin(b) ⇒ cos(2x) = cos(x + x) = cos(x)·cos(x) - sin(x)·sin(x) = cos2(x) -sin2(x) = cos2(x) -(1 -cos2(x)) = 2cos2(x) -1] $\lim_{x \to 0} \frac{(2cos^2(x)-1)-1}{cos(x)-1} = \lim_{x \to 0} \frac{2cos^2(x)-2}{cos(x)-1} = 2·\lim_{x \to 0} \frac{cos^2(x)-1}{cos(x)-1} = 2·\lim_{x \to 0} \frac{(cos(x)-1)(cos(x)+1)}{cos(x)-1} = 2·\lim_{x \to 0} cos(x) + 1 = 2·(1+1) = 4.$
  1. $\lim_{x \to 0}\frac{cos(x)-1}{x^2}$ =[L’Hôpital’s Rule] $\lim_{x \to 0}\frac{-sin(x)}{2x}$ = [There may be instances where we would need to apply L’Hôpital’s Rule multiple times] $\lim_{x \to 0}\frac{-cos(x)}{2} = \frac{-1}{2}.$
  2. $\lim_{x \to 0^+} xln(x)$ =[It seems that L’Hôpital’s Rule does not apply, 0·(-∞)] $\lim_{x \to 0^+} \frac{ln(x)}{1/x}$ = [L’Hôpital’s Rule] $\lim_{x \to 0^+} \frac{1/x}{-1/x^2} = \lim_{x \to 0^+} -x = 0$
  3. $\lim_{x \to ∞} \frac{ln(x)}{x^{1/3}}$ =[L’Hôpital’s Rule, ∞/∞] $\lim_{x \to ∞} \frac{1/x}{1/3·x^{-2/3}} =\lim_{x \to ∞} 3x^{2/3-1} =\lim_{x \to ∞} 3x^{-1/3} = 0.$ In words, ln(x) grows slower than $x^{1/3}$ or any arbitrary positive power of x.
  4. $\lim_{x \to 0+} x^x$ =[Recall that ab=ebln(a)] $\lim_{x \to 0+} e^{xln(x)}$. Notice that $\lim_{x \to 0+} xln(x) = \lim_{x \to 0+} \frac{ln(x)}{1/x} = \lim_{x \to 0+} \frac{1/x}{-1/x^2} = \lim_{x \to 0+} -x = 0 ⇒ \lim_{x \to 0+} x^x = \lim_{x \to 0+} e^{xln(x)} = e^0 = 1.$
  5. $\lim_{x \to 0} \frac{sin(x)}{x}$ =[L’Hôpital’s Rule, 0/0] $\lim_{x \to 0} \frac{cos(x)}{1} = cos(0) = 1.$

Bibliography

This content is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
  1. NPTEL-NOC IITM, Introduction to Galois Theory.
  2. Algebra, Second Edition, by Michael Artin.
  3. LibreTexts, Calculus.
  4. Field and Galois Theory, by Patrick Morandi. Springer.
  5. Michael Penn, and MathMajor.
  6. Contemporary Abstract Algebra, Joseph, A. Gallian.
  7. YouTube’s Andrew Misseldine: Calculus, College Algebra and Abstract Algebra.
  8. Calculus Early Transcendentals: Differential & Multi-Variable Calculus for Social Sciences.
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