Problem Set: Taylor and Maclaurin Series

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

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

5. f(x)=xf(x)=x at a=4a=4

6. f(x)=lnxf(x)=lnx at a=1a=1

7. f(x)=1xf(x)=1x at a=1a=1

8. f(x)=exf(x)=ex at a=1a=1

In the following exercises, verify that the given choice of n in the remainder estimate |Rn|M(n+1)!(xa)n+1|Rn|M(n+1)!(xa)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=310;a=9,n=3

10. [T] (28)13;a=27,n=1(28)13;a=27,n=1

11. [T] sin(6);a=2π,n=5sin(6);a=2π,n=5

12. [T] e2; a=0,n=9a=0,n=9

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

15. Integrate the approximation sinttt36+t5120t75040sinttt36+t5120t75040 evaluated at πt to approximate 10sinπtπtdt10sinπtπtdt.

16. Integrate the approximation ex1+x+x22++x6720ex1+x+x22++x6720 evaluated at −x2 to approximate 10e-x2dx10e-x2dx.

In the following exercises, find the smallest value of n such that the remainder estimate |Rn|M(n+1)!(xa)n+1|Rn|M(n+1)!(xa)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

19. f(x)=e2xf(x)=e2x 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

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 [aR,a+R][aR,a+R] occurs at a or a±Ra±R. Estimate the maximum value of R such that max|f''(z)|2R20.1max|f''(z)|2R20.1 on [aR,a+R][aR,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

23. [T] lnxlnx approximated by x1,a=1x1,a=1

24. [T] cosxcosx approximated by 1,a=01,a=0

In the following exercises, find the Taylor series of the given function centered at the indicated point.

25. x4x4 at a=1a=1

26. 1+x+x2+x31+x+x2+x3 at a=1a=1

27. sinxsinx at a=πa=π

28. cosxcosx at a=2πa=2π

29. sinxsinx at x=π2x=π2

30. cosxcosx at x=π2x=π2

31. exex at a=1a=1

32. exex at a=1a=1

33. 1(x1)21(x1)2 at a=0a=0 (Hint: Differentiate 11x.11x.)

34. 1(x1)31(x1)3 at a=0a=0

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.

36. f(x)=2xf(x)=2x

37. f(x)=x3f(x)=x3

38. f(x)=(x2)2f(x)=(x2)2

39. f(x)=lnxf(x)=lnx

40. f(x)=1xf(x)=1x

41. f(x)=12xx2f(x)=12xx2

42. f(x)=x4x2x21f(x)=x4x2x21

43. f(x)=e-xf(x)=e-x

44. f(x)=e2xf(x)=e2x

[T] In the following exercises, identify the value of x such that the given series n=0ann=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=10n=0anS10=10n=0an.

45. n=01n!n=01n!

46. n=02nn!

47. n=0(1)n(2π)2n(2n)!

48. n=0(1)n(2π)2n+1(2n+1)!

The following exercises make use of the functions S5(x)=xx36+x5120 and C4(x)=1x22+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.

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.

53. [T] Plot exe4(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=xnf(xn)f(xn) approximates solutions of f(x)=0 near the input x0.

  1. If f and g are inverse functions, explain why a solution of g(x)=a is the value f(a)off.
  2. 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.
  3. Explain why the approximate roots of pN(x)2=0 are approximate values of ln(2).

In the following exercises, use the fact that if q(x)=n=1an(xc)n converges in an interval containing c, then limxcq(x)=a0 to evaluate each limit using Taylor series.

55. limx0cosx1x2

56. limx0ln(1x2)x2

57. limx0ex2x21x4

58. limx0+cos(x)12x