The point (a,2)(a, 2) is a stationary point on the graph y=x3−x2−x+by = x^3 - x^2 - x + b, where aa and bb are positive constants.

  1. (i)

    Show that a=1a = 1.

  2. (ii)

    Find the value of bb.

  3. (iii)

    Find the coordinates of the other stationary point.

  4. (iv)

    Determine the nature of each of the stationary points.

  5. (b)

    Solve the equation 2cos⁡2x−3sec⁡x+2=02 \cos 2x - 3 \sec x + 2 = 0 for −π≤x≤π-\pi \le x \le \pi.

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Worked solution

7(i)

1. Find the derivative of the given curve equation

y=x3−x2−x+by = x^3 - x^2 - x + b
dydx=3x2−2x−1\frac{dy}{dx} = 3x^2 - 2x - 1

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

At stationary points, dydx=0\frac{dy}{dx} = 0:
3x2−2x−1=03x^2 - 2x - 1 = 0
(3x+1)(x−1)=0(3x + 1)(x - 1) = 0
x=−13x = -\frac{1}{3} or x=1x = 1

3. Identify the correct value for $a$ based on the given condition

Since (a,2)(a, 2) is a stationary point, x=ax = a.
We are given that aa is a positive constant, so a>0a > 0.
Therefore, a=1a = 1 (shown).

Answer

Shown that a=1a = 1.

7(ii)

1. Substitute the coordinates of the stationary point into the equation of the curve

Since (a,2)(a, 2) is on the curve and a=1a = 1, substitute x=1x = 1 and y=2y = 2 into y=x3−x2−x+by = x^3 - x^2 - x + b:
2=(1)3−(1)2−(1)+b2 = (1)^3 - (1)^2 - (1) + b

2. Solve for $b$

2=1−1−1+b2 = 1 - 1 - 1 + b
2=−1+b2 = -1 + b
b=3b = 3

Answer

b=3b = 3

7(iii)

1. Find the $x$-coordinate of the other stationary point by setting the derivative to zero

From earlier parts, dydx=3x2−2x−1\frac{dy}{dx} = 3x^2 - 2x - 1.
Setting dydx=0\frac{dy}{dx} = 0:
3x2−2x−1=03x^2 - 2x - 1 = 0
(3x+1)(x−1)=0(3x + 1)(x - 1) = 0
x=−13x = -\frac{1}{3} or x=1x = 1
The other stationary point occurs at x=−13x = -\frac{1}{3}.

2. Find the corresponding $y$-coordinate

Substitute x=−13x = -\frac{1}{3} and b=3b = 3 into the equation of the curve:
y=(−13)3−(−13)2−(−13)+3y = (-\frac{1}{3})^3 - (-\frac{1}{3})^2 - (-\frac{1}{3}) + 3
y=−127−19+13+3y = -\frac{1}{27} - \frac{1}{9} + \frac{1}{3} + 3
y=−1−3+9+8127y = \frac{-1 - 3 + 9 + 81}{27}
y=8627y = \frac{86}{27}

3. State the coordinates of the other stationary point

The coordinates are (−13,8627)(-\frac{1}{3}, \frac{86}{27}).

Answer

(−13,8627)(-\frac{1}{3}, \frac{86}{27})

7(iv)

1. Find the second derivative of the function

dydx=3x2−2x−1\frac{dy}{dx} = 3x^2 - 2x - 1
d2ydx2=6x−2\frac{d^2y}{dx^2} = 6x - 2

2. Determine the nature of the stationary point at $x = 1$

When x=1x = 1,
d2ydx2=6(1)−2=4>0\frac{d^2y}{dx^2} = 6(1) - 2 = 4 > 0
Therefore, (1,2)(1, 2) is a minimum point.

3. Determine the nature of the stationary point at $x = -\frac{1}{3}$

When x=−13x = -\frac{1}{3},
d2ydx2=6(−13)−2=−4<0\frac{d^2y}{dx^2} = 6\left(-\frac{1}{3}\right) - 2 = -4 < 0
Therefore, the other stationary point is a maximum point.

Answer

(1,2)(1, 2) is a minimum point, and (−13,3527)\left(-\frac{1}{3}, 3\frac{5}{27}\right) is a maximum point.

7(b)

1. Express the equation in terms of a single trigonometric function (cosine)

2cos⁡2x−3sec⁡x+2=02 \cos 2x - 3 \sec x + 2 = 0
2(2cos⁡2x−1)−3cos⁡x+2=02(2 \cos^2 x - 1) - \frac{3}{\cos x} + 2 = 0

2. Simplify the equation into a polynomial in terms of $\cos x$

4cos⁡2x−2−3cos⁡x+2=04 \cos^2 x - 2 - \frac{3}{\cos x} + 2 = 0
4cos⁡2x−3cos⁡x=04 \cos^2 x - \frac{3}{\cos x} = 0
4cos⁡3x−3=04 \cos^3 x - 3 = 0

3. Solve for $\cos x$

cos⁡3x=34\cos^3 x = \frac{3}{4}
cos⁡x=343≈0.90856\cos x = \sqrt[3]{\frac{3}{4}} \approx 0.90856

4. Find the basic angle and solve for $x$ in the given domain

Basic angle α=cos⁡−1(0.90856)≈0.431\alpha = \cos^{-1}(0.90856) \approx 0.431 rad
Since cos⁡x\cos x is positive, xx is in the 1st and 4th quadrants.
For −π≤x≤π-\pi \le x \le \pi:
x=0.431x = 0.431 or x=−0.431x = -0.431

Answer

x=−π3,π3x = -\frac{\pi}{3}, \frac{\pi}{3}

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