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1.3 Evaluating Limits Analytically

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After this lesson, you will be able to: Evaluate a limit using the properties of limits Develop and use a strategy for finding limits Evaluate a limit using dividing out and rationalizing techniques Evaluate a limit using Squeeze Theorem

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Limits Analytically In the previous lesson, you learned how to find limits numerically and graphically. In this lesson you will be shown how to find them analytically…using algebra or calculus.

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Theorem 1.1 Some Basic Limits Let b and c be real numbers and let n be a positive integer. Examples 1: Think of it graphically… Let As x approaches 3, f(x) approaches 4 Let As x approaches 2, f(x) approaches 2 Let (y scale was adjusted to fit) As x approaches 5, f(x) approaches 125

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Direct Substitution Some limits can be evaluated by direct substituting for x. Direct substitution works on continuous functions. Continuous functions do not have any holes, breaks or gaps. Note: Direct substitution is valid for all polynomial functions and rational functions whose denominators are not zero (or not approaches to zero) as the x approaches to a certain value. Example However

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Theorem 1.2 Properties of Limits Let b and c be real numbers, let n be a positive integer, and let f and g be functions with the following limits: and Scalar multiple: Sum or difference: Product: Quotient: Power:

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Theorem 1.2 Properties of Limits Note: The following assumptions are necessary to those properties of limits. You need provide all the counterexamples to some of the properties if the assumptions are not provided. and Scalar multiple: Sum or difference: Product: Quotient: Power:

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Limit of a Polynomial Function Ans: The limit is 5 Example 2: Since a polynomial function is a continuous function, then we know the limit from the right and left of any number will be the same. Thus, we may use direct substitution.

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If p is a polynomial function and c is a real number, then If p(x) and q(x) are polynomial functions and r(x) = p(x)/q(x) and c is a real number such that q(c) ≠ 0, then Theorem 1.3 Limits of Polynomial and Rational Function

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Limit of a Rational Function Example 3: Ans: 1 Make sure the denominator doesn’t = 0 ! If the denominator had been 0, we would NOT have been able to use direct substitution.

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Theorem 1.4 The Limit of a Function Involving a Radical Let n be a positive integer. The following limit is valid for all c if n is odd, and is valid for c > 0 if n is even.

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Theorem 1.5 The Limit of a Composite Function If f and g are functions such that lim g(x) = L and lim f (x) = f (L), then

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Limit of a Composite Function- part a Example 4: Given a) Find Direct substitution works here

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Limit of a Composite Function - part b b) Find Example 4: Given Direct substitution works here

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Limit of a Composite Function - part c c) Find Example 4: Given From part a, we know that the limit of f( x ) as x approaches 4 is 21

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Theorem 1.6 The Limits of Trig Functions Let c be a real number in the domain of the given trigonometric function

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Limits of Trig Functions Examples 5:

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A Strategy for Finding Limits Try Direct Substitution If the limit of f(x) as x approaches c cannot be evaluated by direct substitution, try to divide out common factors or to rationalize the numerator or the denominator so that direct substitution works. Use a graph or table to reinforce your result.

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Theorem 1.7 Functions That Agree at All But One Point Let c be a real number and let f (x) = g (x) for all x ≠ c in an open interval containing c. If exists, then also exists and

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Example- Factoring Example 6: Direct substitution at this point will give you 0 in the denominator. Using a bit of algebra, we can try to find the limit. Factor Now direct substitution will work Graph on your calculator and use the table to check your result

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Example- Factoring Example 7: Direct substitution results in 0 in the denominator. Try factoring. Now direct substitution will work. Use your calculator to reinforce your result.

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Example Example 8: Direct substitution results in 0 in the denominator. Try factoring. Sum of cubes Not factorable None of the factors can be divided out, so direct substitution still won’t work. The limit DNE. Verify the result on your calculator. The limits from the right and left do not equal each other, thus the limit DNE. Observe how the right limit goes to off to positive infinity and the left limit goes to negative infinity.

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Example- Rationalizing Technique Example 9: Direct substitution results in 0 in the denominator. I see a radical in the numerator. Let’s try rationalizing the numerator. Multiply the top and bottom by the conjugate of the numerator. Note: It was convenient NOT to distribute on the bottom, but you did need to FOIL on the top Now direct substitution will work Go ahead and graph to verify.

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Theorem 1.8 The Squeeze Theorem If h(x) ≤ f (x) ≤ g(x) for all x in an open interval containing c, except possibly at c itself, and if then

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Two Special Trigonometric Limits In your text, read about the Squeeze Theorem on page 65. Following the Squeeze Theorem are the proofs of two special trig limits…I will not expect you to be able to prove the two limits, so you’ll just want to memorize them. The next slide will give them to you and then we’ll use them in a few examples.

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Trig Limits (A star will indicate the need to memorize!!!) Think of this as the limit as “something” approaches 0 of the sine of “something” over the same “something” is equal to 1.

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Example- Using Trig Limits Example 11: Before you decide to even use a special trig limit, make sure that direct substitution won’t work. In this case, direct substitution really won’t work, so let’s try to get this to look like one of those special trig limits. Now, the 5x is like the heart. You will need the bottom to also be 5x in order to use the trig limit. So, multiply the top and bottom by 5. You won’t have changed the fraction. Watch how to do it. This 5 is a constant and can be pulled out in front of the limit. =1 Example 10, page 66, in your text is another example.

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Example Example 12 Direct substitution won’t work. We can use the sine trig limit, but first we’ll have to use some algebra since we need the bottom to be a 3x. To create a 3x on the bottom, we’ll multiply the bottom by 3/2. To be “fair”, we’ll have to multiply the top by 3/2 as well. Watch how I would do it. = 1

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Example Example 13 This example was thrown in to keep you on your toes. Direct substitution works at this point since the bottom of the fraction will not be 0 when you use π/2. Looking at the unit circle, this value is 1

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Example Example 14 This is the 2 nd special trig limit and you should know that this limit is 0. Let’s prove it by using the 1 st trig limit. Again, in this case, it’s best not to distribute on the bottom. You’ll see why it helps to leave it in factored form for now. Use the Pythagorean Trig Identity Be creative Special Trig Limit

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