In the following exercises, find the Taylor polynomials of degree two approximating the given function centered at the given point.
1. f(x)=1+x+x2f(x)=1+x+x2 at a=1a=1
2. f(x)=1+x+x2f(x)=1+x+x2 at a=−1a=−1
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f(−1)=1;f′(−1)=−1;f''(−1)=2;f(x)=1−(x+1)+(x+1)2f(−1)=1;f′(−1)=−1;f''(−1)=2;f(x)=1−(x+1)+(x+1)2
3. f(x)=cos(2x)f(x)=cos(2x) at a=πa=π
4. f(x)=sin(2x)f(x)=sin(2x) at a=π2a=π2
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f′(x)=2cos(2x);f''(x)=−4sin(2x);p2(x)=−2(x−π2)f′(x)=2cos(2x);f''(x)=−4sin(2x);p2(x)=−2(x−π2)
5. f(x)=√xf(x)=√x at a=4a=4
6. f(x)=lnxf(x)=lnx at a=1a=1
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f′(x)=1x;f''(x)=−1x2;p2(x)=0+(x−1)−12(x−1)2f′(x)=1x;f''(x)=−1x2;p2(x)=0+(x−1)−12(x−1)2
7. f(x)=1xf(x)=1x at a=1a=1
8. f(x)=exf(x)=ex at a=1a=1
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p2(x)=e+e(x−1)+e2(x−1)2p2(x)=e+e(x−1)+e2(x−1)2
In the following exercises, verify that the given choice of n in the remainder estimate |Rn|≤M(n+1)!(x−a)n+1|Rn|≤M(n+1)!(x−a)n+1, where M is the maximum value of |f(n+1)(z)||f(n+1)(z)| on the interval between a and the indicated point, yields |Rn|≤11000|Rn|≤11000. Find the value of the Taylor polynomial pn of ff at the indicated point.
9. [T] √10;a=9,n=3√10;a=9,n=3
10. [T] (28)13;a=27,n=1(28)13;a=27,n=1
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d2dx2x13=−29x53≥−0.00092…d2dx2x13=−29x53≥−0.00092… when x≥28x≥28 so the remainder estimate applies to the linear approximation x13≈p1(27)=3+x−2727x13≈p1(27)=3+x−2727, which gives (28)13≈3+127=3.¯037(28)13≈3+127=3.¯¯¯¯¯¯¯¯037, while (28)13≈3.03658(28)13≈3.03658.
11. [T] sin(6);a=2π,n=5sin(6);a=2π,n=5
12. [T] e2; a=0,n=9a=0,n=9
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Using the estimate 21010!<0.00028321010!<0.000283 we can use the Taylor expansion of order 9 to estimate exex at x=2x=2. as e2≈p9(2)=1+2+222+236+⋯+299!=7.3887…e2≈p9(2)=1+2+222+236+⋯+299!=7.3887… whereas e2≈7.3891e2≈7.3891.
13. [T] cos(π5);a=0,n=4cos(π5);a=0,n=4
14. [T] ln(2);a=1,n=1000ln(2);a=1,n=1000
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Since dndxn(lnx)=(−1)n−1(n−1)!xn,R1000≈11001dndxn(lnx)=(−1)n−1(n−1)!xn,R1000≈11001. One has p1000(1)=1000∑n=1(−1)n−1n≈0.6936p1000(1)=1000∑n=1(−1)n−1n≈0.6936 whereas ln(2)≈0.6931⋯ln(2)≈0.6931⋯.
15. Integrate the approximation sint≈t−t36+t5120−t75040sint≈t−t36+t5120−t75040 evaluated at πt to approximate ∫10sinπtπtdt∫10sinπtπtdt.
16. Integrate the approximation ex≈1+x+x22+⋯+x6720ex≈1+x+x22+⋯+x6720 evaluated at −x2 to approximate ∫10e-x2dx∫10e-x2dx.
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∫10(1−x2+x42−x66+x824−x10120+x12720)dx∫10(1−x2+x42−x66+x824−x10120+x12720)dx
=1−133+1510−1742+199⋅24−111120⋅11+113720⋅13≈0.74683=1−133+1510−1742+199⋅24−111120⋅11+113720⋅13≈0.74683 whereas ∫10e-x2dx≈0.74682∫10e-x2dx≈0.74682.
In the following exercises, find the smallest value of n such that the remainder estimate |Rn|≤M(n+1)!(x−a)n+1|Rn|≤M(n+1)!(x−a)n+1, where M is the maximum value of |f(n+1)(z)||f(n+1)(z)| on the interval between a and the indicated point, yields |Rn|≤11000|Rn|≤11000 on the indicated interval.
17. f(x)=sinxf(x)=sinx on [-π,π],a=0[-π,π],a=0
18. f(x)=cosxf(x)=cosx on [−π2,π2],a=0[−π2,π2],a=0
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Since f(n+1)(z)f(n+1)(z) is sinzsinz or coszcosz, we have M=1M=1. Since |x−0|≤π2|x−0|≤π2, we seek the smallest n such that πn+12n+1(n+1)!≤0.001πn+12n+1(n+1)!≤0.001. The smallest such value is n=7n=7. The remainder estimate is R7≤0.00092R7≤0.00092.
19. f(x)=e−2xf(x)=e−2x on [−1,1],a=0[−1,1],a=0
20. f(x)=e-xf(x)=e-x on [−3,3],a=0[−3,3],a=0
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Since f(n+1)(z)=±e-zf(n+1)(z)=±e-z one has M=e3M=e3. Since |x−0|≤3|x−0|≤3, one seeks the smallest n such that 3n+1e3(n+1)!≤0.0013n+1e3(n+1)!≤0.001. The smallest such value is n=14n=14. The remainder estimate is R14≤0.000220R14≤0.000220.
In the following exercises, the maximum of the right-hand side of the remainder estimate |R1|≤max|f''(z)|2R2|R1|≤max|f''(z)|2R2 on [a−R,a+R][a−R,a+R] occurs at a or a±Ra±R. Estimate the maximum value of R such that max|f''(z)|2R2≤0.1max|f''(z)|2R2≤0.1 on [a−R,a+R][a−R,a+R] by plotting this maximum as a function of R.
21. [T] ex approximated by 1+x,a=01+x,a=0
22. [T] sinxsinx approximated by x, a=0a=0
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Since sinxsinx is increasing for small x and since sin''x=-sinxsin''x=-sinx, the estimate applies whenever R2sin(R)≤0.2R2sin(R)≤0.2, which applies up to R=0.596R=0.596.
23. [T] lnxlnx approximated by x−1,a=1x−1,a=1
24. [T] cosxcosx approximated by 1,a=01,a=0
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Since the second derivative of cosxcosx is -cosx-cosx and since cosxcosx is decreasing away from x=0x=0, the estimate applies when R2cosR≤0.2R2cosR≤0.2 or R≤0.447R≤0.447.
In the following exercises, find the Taylor series of the given function centered at the indicated point.
26. 1+x+x2+x31+x+x2+x3 at a=−1a=−1
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(x+1)3−2(x+1)2+2(x+1)(x+1)3−2(x+1)2+2(x+1)
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Values of derivatives are the same as for x=0x=0 so cosx=∞∑n=0(−1)n(x−2π)2n(2n)!cosx=∞∑n=0(−1)n(x−2π)2n(2n)!
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cos(π2)=0,-sin(π2)=−1cos(π2)=0,-sin(π2)=−1 so cosx=∞∑n=0(−1)n+1(x−π2)2n+1(2n+1)!cosx=∞∑n=0(−1)n+1(x−π2)2n+1(2n+1)!, which is also -cos(x−π2)-cos(x−π2).
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The derivatives are f(n)(1)=ef(n)(1)=e so ex=e∞∑n=0(x−1)nn!ex=e∞∑n=0(x−1)nn!.
33. 1(x−1)21(x−1)2 at a=0a=0 (Hint: Differentiate 11−x.11−x.)
34. 1(x−1)31(x−1)3 at a=0a=0
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1(x−1)3=-(12)d2dx211−x=-∞∑n=0((n+2)(n+1)xn2)1(x−1)3=-(12)d2dx211−x=-∞∑n=0((n+2)(n+1)xn2)
35. F(x)=∫x0cos(√t)dt;f(t)=∞∑n=0(−1)ntn(2n)!F(x)=∫x0cos(√t)dt;f(t)=∞∑n=0(−1)ntn(2n)! at a=0a=0 (Note: ff is the Taylor series of cos(√t).cos(√t).)
In the following exercises, compute the Taylor series of each function around x=1x=1.
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2−x=1−(x−1)2−x=1−(x−1)
38. f(x)=(x−2)2f(x)=(x−2)2
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((x−1)−1)2=(x−1)2−2(x−1)+1((x−1)−1)2=(x−1)2−2(x−1)+1
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11−(1−x)=∞∑n=0(−1)n(x−1)n11−(1−x)=∞∑n=0(−1)n(x−1)n
41. f(x)=12x−x2f(x)=12x−x2
42. f(x)=x4x−2x2−1f(x)=x4x−2x2−1
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x∞∑n=02n(1−x)2n=∞∑n=02n(x−1)2n+1+∞∑n=02n(x−1)2nx∞∑n=02n(1−x)2n=∞∑n=02n(x−1)2n+1+∞∑n=02n(x−1)2n
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e2x=e2(x−1)+2=e2∞∑n=02n(x−1)nn!e2x=e2(x−1)+2=e2∞∑n=02n(x−1)nn!
[T] In the following exercises, identify the value of x such that the given series ∞∑n=0an∞∑n=0an is the value of the Maclaurin series of f(x)f(x) at xx. Approximate the value of f(x)f(x) using S10=10∑n=0anS10=10∑n=0an.
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x=e2;S10=34,9134725≈7.3889947
47. ∞∑n=0(−1)n(2π)2n(2n)!
48. ∞∑n=0(−1)n(2π)2n+1(2n+1)!
Show Solution
sin(2π)=0;S10=8.27×10−5
The following exercises make use of the functions S5(x)=x−x36+x5120 and C4(x)=1−x22+x424 on [-π,π].
49. [T] Plot sin2x−(S5(x))2 on [-π,π]. Compare the maximum difference with the square of the Taylor remainder estimate for sinx.
50. [T] Plot cos2x−(C4(x))2 on [-π,π]. Compare the maximum difference with the square of the Taylor remainder estimate for cosx.
Show Solution
The difference is small on the interior of the interval but approaches 1 near the endpoints. The remainder estimate is |R4|=π5120≈2.552.
51. [T] Plot |2S5(x)C4(x)−sin(2x)| on [-π,π].
52. [T] Compare S5(x)C4(x) on [−1,1] to tanx. Compare this with the Taylor remainder estimate for the approximation of tanx by x+x33+2x515.
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The difference is on the order of 10−4 on [−1,1] while the Taylor approximation error is around 0.1 near ±1. The top curve is a plot of tan2x−(S5(x)C4(x))2 and the lower dashed plot shows t2−(S5C4)2.
53. [T] Plot ex−e4(x) where e4(x)=1+x+x22+x36+x424 on [0,2]. Compare the maximum error with the Taylor remainder estimate.
54. (Taylor approximations and root finding.) Recall that Newton’s method xn+1=xn−f(xn)f′(xn) approximates solutions of f(x)=0 near the input x0.
- If f and g are inverse functions, explain why a solution of g(x)=a is the value f(a)off.
- Let pN(x) be the Nth degree Maclaurin polynomial of ex. Use Newton’s method to approximate solutions of pN(x)−2=0 for N=4,5,6.
- Explain why the approximate roots of pN(x)−2=0 are approximate values of ln(2).
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a. Answers will vary. b. The following are the xn values after 10 iterations of Newton’s method to approximation a root of pN(x)−2=0: for N=4,x=0.6939...; for N=5,x=0.6932...; for N=6,x=0.69315...;. (Note: ln(2)=0.69314...) c. Answers will vary.
In the following exercises, use the fact that if q(x)=∞∑n=1an(x−c)n converges in an interval containing c, then limx→cq(x)=a0 to evaluate each limit using Taylor series.
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ln(1−x2)x2→-1
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cos(√x)−12x≈(1−x2+x24!−⋯)−12x→−14
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