A curve has the equation y=3(4−x)3+5y = 3(4 - x)^{3} + 5.

  1. ((i))

    Find the coordinates of the stationary point of the curve.

  2. ((ii))

    Determine the nature of the stationary point.

  3. ((iii))

    Hence sketch the graph of y=3(4−x)3+5y = 3(4 - x)^{3} + 5, showing all the intercepts.

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Worked solution

7((i))

1. Find the derivative of the curve equation

dydx=3⋅3(4−x)2⋅(−1)=−9(4−x)2\frac{dy}{dx} = 3 \cdot 3(4 - x)^2 \cdot (-1) = -9(4 - x)^2

2. Set the derivative to zero to find the $x$-coordinate of the stationary point

For stationary points, dydx=0\frac{dy}{dx} = 0
−9(4−x)2=0-9(4 - x)^2 = 0
4−x=04 - x = 0
x=4x = 4

3. Find the corresponding $y$-coordinate

When x=4x = 4,
y=3(4−4)3+5=5y = 3(4 - 4)^3 + 5 = 5
The coordinates of the stationary point are (4,5)(4, 5).

Answer

(4,5)(4, 5)

7((ii))

1. Recall the first derivative from part (i)

From part (i), dydx=−9(4−x)2\frac{dy}{dx} = -9(4 - x)^2.
The stationary point is at x=4x = 4.

2. Apply the first derivative test around x = 4

Let's test values of xx slightly less than and slightly greater than 4.
When x=3.9x = 3.9, dydx=−9(4−3.9)2=−9(0.1)2=−0.09<0\frac{dy}{dx} = -9(4 - 3.9)^2 = -9(0.1)^2 = -0.09 < 0.
When x=4.1x = 4.1, dydx=−9(4−4.1)2=−9(−0.1)2=−0.09<0\frac{dy}{dx} = -9(4 - 4.1)^2 = -9(-0.1)^2 = -0.09 < 0.

3. Conclude the nature of the stationary point

Since the gradient is negative on both sides of the stationary point, the sign of the gradient does not change.
Therefore, the point (4,5)(4, 5) is a stationary point of inflexion.

Answer

Stationary point of inflexion

7((iii))

1. Find the $y$-intercept

When x=0x = 0,
y=3(4−0)3+5y = 3(4 - 0)^3 + 5
y=3(64)+5=197y = 3(64) + 5 = 197
The yy-intercept is (0,197)(0, 197).

2. Find the $x$-intercept

When y=0y = 0,
3(4−x)3+5=03(4 - x)^3 + 5 = 0
(4−x)3=−53(4 - x)^3 = -\frac{5}{3}
4−x=−5334 - x = \sqrt[3]{-\frac{5}{3}}
x=4+533≈5.19x = 4 + \sqrt[3]{\frac{5}{3}} \approx 5.19
The xx-intercept is (4+533,0)(4 + \sqrt[3]{\frac{5}{3}}, 0).

3. Sketch the graph

Draw a cubic curve with a negative leading coefficient (decreasing overall). The curve passes through the yy-axis at 197197, flattens out at the stationary point of inflexion (4,5)(4, 5), and crosses the xx-axis at approximately 5.195.19.

Answer

A cubic curve passing through (0,197)(0, 197) and (4+533,0)(4 + \sqrt[3]{\frac{5}{3}}, 0), with a stationary point of inflexion at (4,5)(4, 5).

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