Find the coordinates of the stationary points on the graph of y=2x3−9x2+13y = 2x^3 - 9x^2 + 13. Hence, determine the nature of the stationary point(s).

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Worked solution

4(a)

1. Find the first derivative of the function

dydx=ddx(2x3−9x2+13)=6x2−18x\frac{dy}{dx} = \frac{d}{dx}(2x^3 - 9x^2 + 13) = 6x^2 - 18x

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

For stationary points, dydx=0\frac{dy}{dx} = 0:
6x2−18x=06x^2 - 18x = 0
6x(x−3)=06x(x - 3) = 0
x=0 or x=3x = 0 \text{ or } x = 3

3. Find the corresponding $y$-coordinates

When x=0x = 0:
y=2(0)3−9(0)2+13=13y = 2(0)^3 - 9(0)^2 + 13 = 13

When x=3x = 3:
y=2(3)3−9(3)2+13y = 2(3)^3 - 9(3)^2 + 13
y=2(27)−9(9)+13y = 2(27) - 9(9) + 13
y=54−81+13=−14y = 54 - 81 + 13 = -14

4. State the coordinates of the stationary points

The coordinates of the stationary points are (0,13)(0, 13) and (3,−14)(3, -14).

Answer

(0,13)(0, 13) and (3,−14)(3, -14)

4(b)

1. Find the second derivative of the function

From part (a), dydx=6x2−18x\frac{dy}{dx} = 6x^2 - 18x.
d2ydx2=12x−18\frac{d^2y}{dx^2} = 12x - 18

2. Determine the nature of the stationary point at $x = 0$

At x=0x = 0, d2ydx2=12(0)−18=−18\frac{d^2y}{dx^2} = 12(0) - 18 = -18.
Since d2ydx2<0\frac{d^2y}{dx^2} < 0, the point (0,13)(0, 13) is a maximum point.

3. Determine the nature of the stationary point at $x = 3$

At x=3x = 3, d2ydx2=12(3)−18=18\frac{d^2y}{dx^2} = 12(3) - 18 = 18.
Since d2ydx2>0\frac{d^2y}{dx^2} > 0, the point (3,−14)(3, -14) is a minimum point.

Answer

(0,13)(0, 13) is a maximum point and (3,−14)(3, -14) is a minimum point.

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