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Find the equations of the tangent and normal to the curve y = x3 − 3x − 1 at the point (2, 3) and to the curve y = x2 − 2x − 1 when y = -1.

Mathematics

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18 steps

Here are the solutions for questions 5, 6, and 7.

Part 5· 1 step

Given the curve y=x3−3x−1y = x^3 - 3x - 1.

  1. Find the first derivative of the curve.

    dydx=ddx(x3−3x−1)=3x2−3\frac{dy}{dx} = \frac{d}{dx}(x^3 - 3x - 1) = 3x^2 - 3

Part a· 2 steps

Equation of the tangent at the point $(2, 3)$

  1. Calculate the slope of the tangent at x=2x=2.

    mt=3(2)2−3=3(4)−3=12−3=9m_t = 3(2)^2 - 3 = 3(4) - 3 = 12 - 3 = 9
  2. Use the point-slope form y−y1=mt(x−x1)y - y_1 = m_t(x - x_1) with (x1,y1)=(2,3)(x_1, y_1) = (2, 3) and mt=9m_t = 9.

    y−3=9(x−2)y - 3 = 9(x - 2) y−3=9x−18y - 3 = 9x - 18 y=9x−15y = 9x - 15

    The equation of the tangent is y=9x−15\boxed{y = 9x - 15}.

Part b· 2 steps

Equation of the normal at (1,−3)(1, -3)

  1. Calculate the slope of the tangent at x=1x=1.

    mt=3(1)2−3=3−3=0m_t = 3(1)^2 - 3 = 3 - 3 = 0
  2. Calculate the slope of the normal.

    Since the slope of the tangent is 0, the tangent line is horizontal. Therefore, the normal line is vertical.
    A vertical line passing through (1,−3)(1, -3) has the equation x=1x = 1.
    The equation of the normal is x=1\boxed{x = 1}.


Part 6· 8 steps

Find the equations of the tangent and normal to the curve y=x2−2x−1y = x^2 - 2x - 1 when y=−1y = -1.

  1. Find the x-coordinate(s) when y=−1y = -1.

    Substitute y=−1y = -1 into the curve's equation:

    −1=x2−2x−1-1 = x^2 - 2x - 1 0=x2−2x0 = x^2 - 2x

    Factor out xx:

    0=x(x−2)0 = x(x - 2)

    This gives two x-coordinates: x=0x = 0 or x=2x = 2.
    The points are (0,−1)(0, -1) and (2,−1)(2, -1).

  2. Find the first derivative of the curve.

    dydx=ddx(x2−2x−1)=2x−2\frac{dy}{dx} = \frac{d}{dx}(x^2 - 2x - 1) = 2x - 2

    At point (0,−1)(0, -1):

  3. Calculate the slope of the tangent at x=0x=0.

    mt=2(0)−2=−2m_t = 2(0) - 2 = -2
  4. Equation of the tangent at (0,−1)(0, -1).

    Using y−y1=mt(x−x1)y - y_1 = m_t(x - x_1):

    y−(−1)=−2(x−0)y - (-1) = -2(x - 0) y+1=−2xy + 1 = -2x y=−2x−1y = -2x - 1

    The equation of the tangent is y=−2x−1\boxed{y = -2x - 1}.

  5. Equation of the normal at (0,−1)(0, -1).

    The slope of the normal mn=−1mt=−1−2=12m_n = -\frac{1}{m_t} = -\frac{1}{-2} = \frac{1}{2}.
    Using y−y1=mn(x−x1)y - y_1 = m_n(x - x_1):

    y−(−1)=12(x−0)y - (-1) = \frac{1}{2}(x - 0) y+1=12xy + 1 = \frac{1}{2}x y=12x−1y = \frac{1}{2}x - 1

    The equation of the normal is y=12x−1\boxed{y = \frac{1}{2}x - 1}.

    At point (2,−1)(2, -1):

  6. Calculate the slope of the tangent at x=2x=2.

    mt=2(2)−2=4−2=2m_t = 2(2) - 2 = 4 - 2 = 2
  7. Equation of the tangent at (2,−1)(2, -1).

    Using y−y1=mt(x−x1)y - y_1 = m_t(x - x_1):

    y−(−1)=2(x−2)y - (-1) = 2(x - 2) y+1=2x−4y + 1 = 2x - 4 y=2x−5y = 2x - 5

    The equation of the tangent is y=2x−5\boxed{y = 2x - 5}.

  8. Equation of the normal at (2,−1)(2, -1).

    The slope of the normal mn=−1mt=−12m_n = -\frac{1}{m_t} = -\frac{1}{2}.
    Using y−y1=mn(x−x1)y - y_1 = m_n(x - x_1):

    y−(−1)=−12(x−2)y - (-1) = -\frac{1}{2}(x - 2) y+1=−12x+1y + 1 = -\frac{1}{2}x + 1 y=−12xy = -\frac{1}{2}x

    The equation of the normal is y=−12x\boxed{y = -\frac{1}{2}x}.


Part 7· 5 steps

Determine the point on the curve y=12x2+4y = \frac{1}{2}x^2 + 4 at which the gradient is 8. Hence, find the equation of the normal to the curve at this point.

  1. Find the first derivative of the curve (gradient function).

    dydx=ddx(12x2+4)=x\frac{dy}{dx} = \frac{d}{dx}\left(\frac{1}{2}x^2 + 4\right) = x
  2. Find the x-coordinate where the gradient is 8.

    Set dydx=8\frac{dy}{dx} = 8:

    x=8x = 8
  3. Find the y-coordinate of the point.

    Substitute x=8x=8 into the original equation:

    y=12(8)2+4=12(64)+4=32+4=36y = \frac{1}{2}(8)^2 + 4 = \frac{1}{2}(64) + 4 = 32 + 4 = 36

    The point is (8,36)(8, 36).

  4. Determine the slope of the normal.

    The gradient (slope of the tangent) at this point is mt=8m_t = 8.
    The slope of the normal mn=−1mt=−18m_n = -\frac{1}{m_t} = -\frac{1}{8}.

  5. Find the equation of the normal.

    Using the point-slope form y−y1=mn(x−x1)y - y_1 = m_n(x - x_1) with (x1,y1)=(8,36)(x_1, y_1) = (8, 36) and mn=−18m_n = -\frac{1}{8}.

    y−36=−18(x−8)y - 36 = -\frac{1}{8}(x - 8) y−36=−18x+1y - 36 = -\frac{1}{8}x + 1 y=−18x+37y = -\frac{1}{8}x + 37

    The equation of the normal is y=−18x+37\boxed{y = -\frac{1}{8}x + 37}.

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