High School

Find an equation for the tangent line to the graph of \( f \) at the given point.

Given:
\[ f(x) = (4x^3 + 6)^2 \]
Point: \((-1, 4)\)

Select one:

A. \( y = 80x + 84 \)
B. \( y = -80x + 84 \)
C. \( y = 80x - 84 \)
D. \( y = 20x - 84 \)
E. \( y = 20x + 84 \)

Answer :

To find the equation of the tangent line to the graph of the function [tex]f(x) = (4x^3 + 6)^2[/tex] at the point [tex](-1, 4)[/tex], we need to follow these steps:


  1. Differentiate the Function:
    First, we need to find the derivative [tex]f'(x)[/tex] of the function [tex]f(x) = (4x^3 + 6)^2[/tex]. We will use the chain rule for differentiation:

    [tex]u = 4x^3 + 6 \quad \Rightarrow \quad f(x) = u^2[/tex]

    Differentiate [tex]u[/tex] with respect to [tex]x[/tex]:

    [tex]\frac{du}{dx} = 12x^2[/tex]

    Then, apply the chain rule to differentiate [tex]f(x)[/tex]:

    [tex]f'(x) = 2u \cdot \frac{du}{dx} = 2(4x^3 + 6) \cdot 12x^2 = 24x^2 (4x^3 + 6)[/tex]


  2. Evaluate the Derivative at [tex]x = -1[/tex]:
    Now, substitute [tex]x = -1[/tex] into the derivative to find the slope [tex]m[/tex] of the tangent line:

    [tex]f'(-1) = 24(-1)^2 (4(-1)^3 + 6) = 24 \cdot 1 \cdot ( -4 + 6) = 24 \cdot 2 = 48[/tex]


  3. Equation of the Tangent Line:
    With the slope [tex]m = 48[/tex] and the point [tex](-1, 4)[/tex], use the point-slope form of the line equation:

    [tex]y - y_1 = m(x - x_1)[/tex]

    Substituting [tex]y_1 = 4[/tex], [tex]m = 48[/tex], and [tex]x_1 = -1[/tex]:

    [tex]y - 4 = 48(x + 1)[/tex]

    Simplify the equation:

    [tex]y - 4 = 48x + 48[/tex]
    [tex]y = 48x + 52[/tex]



Unfortunately, none of the multiple-choice options matches the equation we derived, suggesting there might be a miscalculation or a typo in the options provided in the question. Since there is no exact match:

Based on our calculations, the equation of the tangent line should be [tex]y = 48x + 52[/tex], but if we need to choose from the provided options, further verification is necessary. In any case, revisit optional calculations or seek clarification to ensure complete accuracy.