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Calculus help (finding equation of a tangent line)?

Write an equation of the line tangent to the graph of g at x=3.

f(x) = (squareroot)4x-3

g(x) = f(x)/x

The slope is supposed to be -1/9, but I keep getting 1/3 and it's driving me nuts.

2 Answers

Relevance
  • Niall
    Lv 7
    7 years ago

    Sub in the given x coordinate to find the y coordinate:

    g(x) = √9/3 = 1

    Recall that x = √x^2:

    g(x) = (4x^-1 - 3x^-2)^1/2

    Differentiate using the chain rule:

    g'(x) = 1/2 * (4x^-1 - 3x^-2)^-1/2 * (-4x^-2 + 6x^-3)

    g'(x) = 1/2 * (4/x - 3/x^2)^1/2 * (-4/x^2 + 6/x^3)

    Sub in the given coordinates to find the slope of the tangent line:

    g'(x) = 1/2 * 1 * -2/9

    g'(x) = -1/9

    Now that you have the slope of the tangent line, you can find its equation:

    y - 1 = -1/9(x - 3)

    -9y + 9 = x - 3

    x + 9y - 12 = 0

  • Rahul
    Lv 4
    7 years ago

    g(x) = sqrt(4x - 3)/x

    g'(x) = 4/(2 x sqrt(4x - 3)) - sqrt(4x - 3)/x^2 = (2/9) - 1/3 = (2 - 3)/9 = -1/9

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