The equation of a curve is y=x2(6−x)y = x^2(6 - x).

  1. (i)

    Find the coordinates of the stationary points of the curve.

  2. (ii)

    Use the second derivative test to determine the nature of each of these points.

  3. (iii)

    Find the range of values of xx for which yy is increasing.

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Worked solution

7(i)

1. Expand the equation of the curve and find the first derivative

y=x2(6−x)=6x2−x3y = x^2(6 - x) = 6x^2 - x^3

dydx=12x−3x2\frac{dy}{dx} = 12x - 3x^2

2. Set the derivative to zero to find the $x$-coordinates of the stationary points

For stationary points, dydx=0\frac{dy}{dx} = 0:
12x−3x2=012x - 3x^2 = 0
3x(4−x)=03x(4 - x) = 0
x=0x = 0 or x=4x = 4

3. Find the corresponding $y$-coordinates

When x=0x = 0, y=02(6−0)=0y = 0^2(6 - 0) = 0
When x=4x = 4, y=42(6−4)=16(2)=32y = 4^2(6 - 4) = 16(2) = 32

4. State the coordinates of the stationary points

The coordinates of the stationary points are (0,0)(0, 0) and (4,32)(4, 32).

Answer

(0,0)(0, 0) and (4,32)(4, 32)

7(ii)

1. Find the second derivative of the curve

From part (i), dydx=12x−3x2\frac{dy}{dx} = 12x - 3x^2.

d2ydx2=12−6x\frac{d^2y}{dx^2} = 12 - 6x

2. Apply the second derivative test to the first stationary point

At x=0x = 0:
d2ydx2=12−6(0)=12\frac{d^2y}{dx^2} = 12 - 6(0) = 12
Since 12>012 > 0, the point (0,0)(0, 0) is a minimum point.

3. Apply the second derivative test to the second stationary point

At x=4x = 4:
d2ydx2=12−6(4)=−12\frac{d^2y}{dx^2} = 12 - 6(4) = -12
Since −12<0-12 < 0, the point (4,32)(4, 32) is a maximum point.

Answer

(0,0)(0, 0) is a minimum point and (4,32)(4, 32) is a maximum point.

7(iii)

1. Set up the condition for the function to be increasing

For yy to be increasing, dydx>0\frac{dy}{dx} > 0.
From part (i), dydx=12x−3x2\frac{dy}{dx} = 12x - 3x^2.
12x−3x2>012x - 3x^2 > 0

2. Solve the quadratic inequality

3x(4−x)>03x(4 - x) > 0
The roots are x=0x = 0 and x=4x = 4. Since the coefficient of x2x^2 is negative, the expression is positive between the roots.
0<x<40 < x < 4

Answer

0<x<40 < x < 4

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