The equation of a curve is y=2x3+6x2−18x+5y = 2x^3 + 6x^2 - 18x + 5.

  1. (a)

    Find the coordinates of the stationary points and determine the nature of these stationary points.

  2. (b)

    Find the minimum gradient of the curve and the value of xx when the minimum gradient occurs.

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Worked solution

10(a)

1. Find the first derivative of the curve equation.

dydx=ddx(2x3+6x2−18x+5)=6x2+12x−18\frac{dy}{dx} = \frac{d}{dx}(2x^3 + 6x^2 - 18x + 5) = 6x^2 + 12x - 18

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

For stationary points, dydx=0\frac{dy}{dx} = 0:
6x2+12x−18=06x^2 + 12x - 18 = 0
x2+2x−3=0x^2 + 2x - 3 = 0
(x+3)(x−1)=0(x + 3)(x - 1) = 0
x=−3x = -3 or x=1x = 1

3. Find the corresponding $y$-coordinates.

When x=−3x = -3, y=2(−3)3+6(−3)2−18(−3)+5=−54+54+54+5=59y = 2(-3)^3 + 6(-3)^2 - 18(-3) + 5 = -54 + 54 + 54 + 5 = 59
When x=1x = 1, y=2(1)3+6(1)2−18(1)+5=2+6−18+5=−5y = 2(1)^3 + 6(1)^2 - 18(1) + 5 = 2 + 6 - 18 + 5 = -5
The coordinates of the stationary points are (−3,59)(-3, 59) and (1,−5)(1, -5).

4. Find the second derivative to determine the nature of the stationary points.

d2ydx2=ddx(6x2+12x−18)=12x+12\frac{d^2y}{dx^2} = \frac{d}{dx}(6x^2 + 12x - 18) = 12x + 12

5. Apply the second derivative test to each stationary point.

When x=−3x = -3, d2ydx2=12(−3)+12=−24<0\frac{d^2y}{dx^2} = 12(-3) + 12 = -24 < 0.
Therefore, (−3,59)(-3, 59) is a maximum point.

When x=1x = 1, d2ydx2=12(1)+12=24>0\frac{d^2y}{dx^2} = 12(1) + 12 = 24 > 0.
Therefore, (1,−5)(1, -5) is a minimum point.

Answer

(−3,59)(-3, 59) is a maximum point, (1,−5)(1, -5) is a minimum point.

10(b)

1. Find the expression for the gradient of the curve.

Let the gradient be mm.
m=dydx=6x2+12x−18m = \frac{dy}{dx} = 6x^2 + 12x - 18

2. Differentiate the gradient expression and equate to zero to find the value of $x$ for the minimum gradient.

dmdx=d2ydx2=12x+12\frac{dm}{dx} = \frac{d^2y}{dx^2} = 12x + 12
For a minimum gradient, dmdx=0\frac{dm}{dx} = 0:
12x+12=012x + 12 = 0
x=−1x = -1

3. Calculate the minimum gradient.

Substitute x=−1x = -1 into the gradient expression:
m=6(−1)2+12(−1)−18m = 6(-1)^2 + 12(-1) - 18
m=6−12−18=−24m = 6 - 12 - 18 = -24
(Note: d2mdx2=12>0\frac{d^2m}{dx^2} = 12 > 0, confirming it is a minimum.)

Answer

Minimum gradient is −24-24, which occurs when x=−1x = -1.

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