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§8.1 Extending local linearization (continued)

在 $a = 0$ 附近的切线对函数 $f(x) = e^x$(在 $a = 0$ 附近)的近似效果如何?

1. As the degree of the approximation increases

As the degree of the approximation increases

2. Summary

Summary

3. Quadratic error on the right flank

For f(x)=exf(x) = e^x and T2(x)=1+x+12x2T_2(x) = 1+x+\frac{1}{2}x^2, the absolute error at x=1x = 1 is approximately

4. Cubic error on the left flank

For the cubic T3(x)=1+x+12x2+16x3T_3(x) = 1+x+\frac{1}{2}x^2+\frac{1}{6}x^3, the absolute error ∣f(−1)−T3(−1)∣|f(-1)-T_3(-1)| is approximately

5. Stretching the window shows the seams

When the spreadsheet window widens to something like −2<x<2-2 \lt x \lt 2, the polynomial approximations show that

6. Reading the degree ladder from the table

Scanning the spreadsheet columns at a fixed xx while the degree climbs from 11 to 44 shows the error

7. A quadratic stand-in for a cubic

For f(x)=13x3+14x2−2x−1f(x) = \frac{1}{3}x^3+\frac{1}{4}x^2-2x-1 near a=0a = 0, the quadratic approximation is

8. The quadratic error on the left

For T2(x)=1+x+12x2T_2(x) = 1+x+\frac{1}{2}x^2 approximating exe^x, the absolute error at x=−1x = -1 is about 0.132120.13212. Report this error.

9. Quartic precision at one

For T4(x)=1+x+12x2+16x3+124x4T_4(x) = 1+x+\frac{1}{2}x^2+\frac{1}{6}x^3+\frac{1}{24}x^4, the absolute error at x=1x = 1 is approximately 0.009950.00995. Report this value.

10. A linear model for a cubic function

For f(x)=13x3+14x2−2x−1f(x) = \frac{1}{3}x^3+\frac{1}{4}x^2-2x-1, the linear approximation at a=0a=0 is T1(x)=−1−2xT_1(x) = -1-2x. Compute T1(0.5)T_1(0.5).

11. Cubics reproduce themselves

For the cubic function f(x)=13x3+14x2−2x−1f(x) = \frac{1}{3}x^3+\frac{1}{4}x^2-2x-1, the degree 33 Taylor polynomial at a=0a = 0 equals ff itself.

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