A curve is such that dydx=8x2−2.\frac{dy}{dx} = \frac{8}{x^{2}} - 2.

  1. ((i))

    Given that the curve passes through the point (1, 5), find the equation of the curve.

  2. ((ii))

    Find the x--coordinates of the stationary points of the curve.

  3. ((iii))

    Obtain an expression for d2ydx2\frac{d^{2}y}{dx^{2}} and hence, or otherwise, determine the nature of each stationary point.

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Worked solution

6((i))

1. Integrate the given derivative to find the general equation of the curve

dydx=8x−2−2\frac{dy}{dx} = 8x^{-2} - 2
y=∫(8x−2−2) dxy = \int (8x^{-2} - 2) \, dx
y=8x−1−1−2x+cy = \frac{8x^{-1}}{-1} - 2x + c
y=−8x−2x+cy = -\frac{8}{x} - 2x + c

2. Substitute the given point to find the constant of integration

Substitute x=1,y=5x = 1, y = 5:
5=−81−2(1)+c5 = -\frac{8}{1} - 2(1) + c
5=−10+c5 = -10 + c
c=15c = 15

3. State the final equation of the curve

y=−8x−2x+15y = -\frac{8}{x} - 2x + 15

Answer

y=−8x−2x+15y = -\frac{8}{x} - 2x + 15

6((ii))

1. Set the derivative to zero to find stationary points

For stationary points, dydx=0\frac{dy}{dx} = 0.
8x2−2=0\frac{8}{x^{2}} - 2 = 0

2. Solve the equation for $x$

8x2=2\frac{8}{x^{2}} = 2
2x2=82x^{2} = 8
x2=4x^{2} = 4
x=2 or x=−2x = 2 \text{ or } x = -2

Answer

x=2x = 2 or x=−2x = -2

6((iii))

1. Differentiate $\frac{dy}{dx}$ to find $\frac{d^{2}y}{dx^{2}}$.

dydx=8x−2−2\frac{dy}{dx} = 8x^{-2} - 2
d2ydx2=−16x−3=−16x3\frac{d^{2}y}{dx^{2}} = -16x^{-3} = -\frac{16}{x^{3}}

2. Find the stationary points from part (ii) to test.

From (ii), dydx=0  ⟹  8x2−2=0  ⟹  x2=4  ⟹  x=2\frac{dy}{dx} = 0 \implies \frac{8}{x^2} - 2 = 0 \implies x^2 = 4 \implies x = 2 or x=−2x = -2.

3. Evaluate the second derivative at $x = 2$ to determine its nature.

When x=2x = 2, d2ydx2=−1623=−2<0\frac{d^{2}y}{dx^{2}} = -\frac{16}{2^{3}} = -2 < 0.
Therefore, the stationary point at x=2x = 2 is a maximum.

4. Evaluate the second derivative at $x = -2$ to determine its nature.

When x=−2x = -2, d2ydx2=−16(−2)3=2>0\frac{d^{2}y}{dx^{2}} = -\frac{16}{(-2)^{3}} = 2 > 0.
Therefore, the stationary point at x=−2x = -2 is a minimum.

Answer

d2ydx2=−16x3\frac{d^{2}y}{dx^{2}} = -\frac{16}{x^{3}}. The stationary point at x=2x = 2 is a maximum, and the stationary point at x=−2x = -2 is a minimum.

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