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Given that y = (x−2)(2x2−3x−5), find dy/dx and find the gradient of the tangents to the curve where it cuts the y-axis.

Mathematics

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Answer

1
Graph of y = (x − 2)(2x² − 3x − 5)

Use the left and right arrow keys to trace the curve, up and down to jump between key points, and plus and minus to zoom.

Key points of y = (x − 2)(2x² − 3x − 5)
Pointxy
x-intercept−10
y-intercept010
Maximum0.0710.04
x-intercept20
Minimum2.26−0.41
x-intercept2.50

Step-by-step solution

4 steps
  1. Expand the given function y=(x−2)(2x2−3x−5)y = (x-2)(2x^2-3x-5).

    y=x(2x2−3x−5)−2(2x2−3x−5)y = x(2x^2-3x-5) - 2(2x^2-3x-5) y=2x3−3x2−5x−4x2+6x+10y = 2x^3 - 3x^2 - 5x - 4x^2 + 6x + 10 y=2x3−7x2+x+10y = 2x^3 - 7x^2 + x + 10
  2. Differentiate yy with respect to xx to find dydx\frac{dy}{dx}.

    Using the power rule for differentiation, ddx(axn)=naxn−1\frac{d}{dx}(ax^n) = nax^{n-1}:

    dydx=ddx(2x3)−ddx(7x2)+ddx(x)+ddx(10)\frac{dy}{dx} = \frac{d}{dx}(2x^3) - \frac{d}{dx}(7x^2) + \frac{d}{dx}(x) + \frac{d}{dx}(10) dydx=2(3)x3−1−7(2)x2−1+1x1−1+0\frac{dy}{dx} = 2(3)x^{3-1} - 7(2)x^{2-1} + 1x^{1-1} + 0 dydx=6x2−14x+1\frac{dy}{dx} = 6x^2 - 14x + 1
  3. Find the point where the curve cuts the y-axis.

    The curve cuts the y-axis when x=0x=0.
    Substitute x=0x=0 into the original equation for yy:

    y=(0−2)(2(0)2−3(0)−5)y = (0-2)(2(0)^2-3(0)-5) y=(−2)(−5)y = (-2)(-5) y=10y = 10

    So, the curve cuts the y-axis at the point (0,10)(0, 10).

  4. Find the gradient of the tangent at the point where the curve cuts the y-axis.

    Substitute x=0x=0 into the derivative dydx\frac{dy}{dx} to find the gradient:

    dydx∣x=0=6(0)2−14(0)+1\frac{dy}{dx}\Big|_{x=0} = 6(0)^2 - 14(0) + 1 dydx∣x=0=0−0+1\frac{dy}{dx}\Big|_{x=0} = 0 - 0 + 1 dydx∣x=0=1\frac{dy}{dx}\Big|_{x=0} = 1

    The gradient of the tangent to the curve where it cuts the y-axis is 1\boxed{1}.

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