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Given f(x) = -3x³ + 30x and a = -3.

Mathematics

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Answer

2.1

L(x)=−51x−162L(x) = -51x - 162

Graph of f(x) = −3x³ + 30x

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Key points of f(x) = −3x³ + 30x
Pointxy
x-intercept−3.160
Minimum−1.83−36.51
x-intercept00
Maximum1.8336.51
x-intercept3.160

Step-by-step solution

7 steps

Part 2.1· 3 steps

Use Taylor method to linearize f(x)=−3x3+30xf(x) = -3x^3 + 30x when x=−3x = -3.

  1. Find f(a)f(a) and f′(x)f'(x).

    The linearization of f(x)f(x) at x=ax=a is given by L(x)=f(a)+f′(a)(x−a)L(x) = f(a) + f'(a)(x-a).
    Given f(x)=−3x3+30xf(x) = -3x^3 + 30x and a=−3a = -3.

    First, calculate f(a)f(a):

    f(−3)=−3(−3)3+30(−3)=−3(−27)−90=81−90=−9f(-3) = -3(-3)^3 + 30(-3) = -3(-27) - 90 = 81 - 90 = -9

    Next, find the first derivative f′(x)f'(x):

    f′(x)=ddx(−3x3+30x)=−9x2+30f'(x) = \frac{d}{dx}(-3x^3 + 30x) = -9x^2 + 30
  2. Calculate f′(a)f'(a).

    Substitute a=−3a=-3 into f′(x)f'(x):

    f′(−3)=−9(−3)2+30=−9(9)+30=−81+30=−51f'(-3) = -9(-3)^2 + 30 = -9(9) + 30 = -81 + 30 = -51
  3. Form the linearization L(x)L(x).

    Substitute f(−3)f(-3) and f′(−3)f'(-3) into the linearization formula:

    L(x)=f(−3)+f′(−3)(x−(−3))L(x) = f(-3) + f'(-3)(x - (-3)) L(x)=−9+(−51)(x+3)L(x) = -9 + (-51)(x + 3) L(x)=−9−51x−153L(x) = -9 - 51x - 153 L(x)=−51x−162L(x) = -51x - 162

    The linearization of f(x)f(x) at x=−3x=-3 is:

    L(x)=−51x−162\boxed{L(x) = -51x - 162}

Part 2.2· 4 steps

If R(x)=22+25(x−2)+11(x−2)2+2(x−2)3R(x) = 22 + 25(x-2) + 11(x-2)^2 + 2(x-2)^3, prove that R(x)R(x) is the Taylor polynomial for f(x)=2x3−x2+5xf(x) = 2x^3 - x^2 + 5x, where a=2a = 2.

To prove R(x)R(x) is the Taylor polynomial for f(x)f(x) at a=2a=2, we need to show that the coefficients of R(x)R(x) match the Taylor series coefficients f(k)(a)k!\frac{f^{(k)}(a)}{k!}.
The Taylor polynomial of degree 3 centered at aa is:

P3(x)=f(a)+f′(a)(x−a)+f′′(a)2!(x−a)2+f′′′(a)3!(x−a)3P_3(x) = f(a) + f'(a)(x-a) + \frac{f''(a)}{2!}(x-a)^2 + \frac{f'''(a)}{3!}(x-a)^3

Given f(x)=2x3−x2+5xf(x) = 2x^3 - x^2 + 5x and a=2a=2.

  1. Calculate f(a)f(a).

    f(2)=2(2)3−(2)2+5(2)=2(8)−4+10=16−4+10=22f(2) = 2(2)^3 - (2)^2 + 5(2) = 2(8) - 4 + 10 = 16 - 4 + 10 = 22

    This matches the constant term in R(x)R(x).

  2. Calculate f′(a)f'(a).

    First, find the first derivative f′(x)f'(x):

    f′(x)=ddx(2x3−x2+5x)=6x2−2x+5f'(x) = \frac{d}{dx}(2x^3 - x^2 + 5x) = 6x^2 - 2x + 5

    Now, evaluate f′(2)f'(2):

    f′(2)=6(2)2−2(2)+5=6(4)−4+5=24−4+5=25f'(2) = 6(2)^2 - 2(2) + 5 = 6(4) - 4 + 5 = 24 - 4 + 5 = 25

    This matches the coefficient of (x−2)(x-2) in R(x)R(x).

  3. Calculate f′′(a)f''(a).

    First, find the second derivative f′′(x)f''(x):

    f′′(x)=ddx(6x2−2x+5)=12x−2f''(x) = \frac{d}{dx}(6x^2 - 2x + 5) = 12x - 2

    Now, evaluate f′′(2)f''(2):

    f′′(2)=12(2)−2=24−2=22f''(2) = 12(2) - 2 = 24 - 2 = 22

    The Taylor coefficient for (x−a)2(x-a)^2 is f′′(a)2!\frac{f''(a)}{2!}:

    f′′(2)2!=222=11\frac{f''(2)}{2!} = \frac{22}{2} = 11

    This matches the coefficient of (x−2)2(x-2)^2 in R(x)R(x).

  4. Calculate f′′′(a)f'''(a).

    First, find the third derivative f′′′(x)f'''(x):

    f′′′(x)=ddx(12x−2)=12f'''(x) = \frac{d}{dx}(12x - 2) = 12

    Now, evaluate f′′′(2)f'''(2):

    f′′′(2)=12f'''(2) = 12

    The Taylor coefficient for (x−a)3(x-a)^3 is f′′′(a)3!\frac{f'''(a)}{3!}:

    f′′′(2)3!=126=2\frac{f'''(2)}{3!} = \frac{12}{6} = 2

    This matches the coefficient of (x−2)3(x-2)^3 in R(x)R(x).

    Since all calculated Taylor coefficients f(2)f(2), f′(2)f'(2), f′′(2)2!\frac{f''(2)}{2!}, and f′′′(2)3!\frac{f'''(2)}{3!} match the coefficients in R(x)R(x), R(x)R(x) is indeed the Taylor polynomial for f(x)f(x) centered at a=2a=2.

Part 2.3

What makes you think that R(x)R(x) is the Taylor polynomial of f(x)f(x) in 2.2.

The reason R(x)R(x) is considered the Taylor polynomial of f(x)f(x) in 2.2 is that its coefficients are precisely the Taylor coefficients of f(x)f(x) evaluated at a=2a=2. Specifically, for each term (x−2)k(x-2)^k in R(x)R(x), its coefficient is equal to f(k)(2)k!\frac{f^{(k)}(2)}{k!}, where f(k)(2)f^{(k)}(2) is the kk-th derivative of f(x)f(x) evaluated at x=2x=2.

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