The diagram shows part of the curve y=27+8(x−2)3y = 27 + \frac{8}{(x-2)^3} which crosses the axes at points AA and BB.

  1. (a)

    Explain why the curve does not have a stationary point.

  2. (b)

    Find the coordinates of AA and of BB.

  3. (c)

    Find the equation of the normal to the curve at BB.

  4. (d)

    The normal to the curve at BB cuts the xx-axis at CC.
    Find the area of triangle BOCBOC.

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Worked solution

8(a)

1. Find the first derivative of the curve equation

y=27+8(x−2)−3y = 27 + 8(x-2)^{-3}
dydx=8(−3)(x−2)−4=−24(x−2)4\frac{dy}{dx} = 8(-3)(x-2)^{-4} = -\frac{24}{(x-2)^4}

2. State the condition for a stationary point and explain why it is not met

For a stationary point, dydx=0\frac{dy}{dx} = 0.
However, −24(x−2)4=0-\frac{24}{(x-2)^4} = 0 has no solution because the numerator is −24≠0-24 \neq 0.
Therefore, the curve does not have a stationary point.

Answer

Since dydx=−24(x−2)4≠0\frac{dy}{dx} = -\frac{24}{(x-2)^4} \neq 0 for all real xx, the curve has no stationary point.

8(b)

1. Find the coordinates of $A$ by setting $y = 0$

At AA, y=0y = 0:
0=27+8(x−2)30 = 27 + \frac{8}{(x-2)^3}
8(x−2)3=−27\frac{8}{(x-2)^3} = -27
(x−2)3=−827(x-2)^3 = -\frac{8}{27}
x−2=−23x - 2 = -\frac{2}{3}
x=43x = \frac{4}{3}
Coordinates of AA are (43,0)\left(\frac{4}{3}, 0\right).

2. Find the coordinates of $B$ by setting $x = 0$

At BB, x=0x = 0:
y=27+8(0−2)3y = 27 + \frac{8}{(0-2)^3}
y=27+8−8y = 27 + \frac{8}{-8}
y=27−1=26y = 27 - 1 = 26
Coordinates of BB are (0,26)(0, 26).

Answer

A(43,0)A\left(\frac{4}{3}, 0\right), B(0,26)B(0, 26)

8(c)

1. Find the coordinates of point $B$ (the $y$-intercept).

At BB, x=0x = 0.
y=27+8(0−2)3=27+8−8=26y = 27 + \frac{8}{(0-2)^3} = 27 + \frac{8}{-8} = 26
So BB is (0,26)(0, 26).

2. Find the derivative $\frac{dy}{dx}$ and evaluate it at $x = 0$ to find the gradient of the tangent.

y=27+8(x−2)−3y = 27 + 8(x-2)^{-3}
dydx=−24(x−2)−4=−24(x−2)4\frac{dy}{dx} = -24(x-2)^{-4} = -\frac{24}{(x-2)^4}
At x=0x = 0, gradient of tangent mt=−24(−2)4=−2416=−32m_t = -\frac{24}{(-2)^4} = -\frac{24}{16} = -\frac{3}{2}

3. Find the gradient of the normal and the equation of the normal.

Gradient of normal mn=−1mt=23m_n = -\frac{1}{m_t} = \frac{2}{3}
Equation of normal at B(0,26)B(0, 26):
y−26=23(x−0)y - 26 = \frac{2}{3}(x - 0)
y=23x+26y = \frac{2}{3}x + 26

Answer

y=23x+26y = \frac{2}{3}x + 26

8(d)

1. Find the coordinates of $C$ using the equation of the normal from part (c)

From part (c), the equation of the normal at B(0,26)B(0, 26) is y=23x+26y = \frac{2}{3}x + 26.
At CC, the normal cuts the xx-axis, so y=0y = 0.
0=23x+260 = \frac{2}{3}x + 26
23x=−26\frac{2}{3}x = -26
x=−39x = -39
So, CC is (−39,0)(-39, 0).

2. Calculate the area of triangle $BOC$

The vertices of triangle BOCBOC are B(0,26)B(0, 26), O(0,0)O(0, 0), and C(−39,0)C(-39, 0).
The base of the triangle along the xx-axis is OC=39OC = 39 units.
The perpendicular height along the yy-axis is OB=26OB = 26 units.
Area=12×base×height\text{Area} = \frac{1}{2} \times \text{base} \times \text{height}
Area=12×39×26\text{Area} = \frac{1}{2} \times 39 \times 26
Area=507\text{Area} = 507

Answer

507507

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