The graph shows part of the curve y=−3(2x−4)−1/3y = -3(2x-4)^{-1/3}. The point P(−2, 1.5)P(-2,\ 1.5) lies on the curve and the normal to the curve at PP meets the x-axis at QQ.

  1. (a)

    Find the coordinates of QQ.

  2. (b)

    Find the area of the shaded region bounded by the curve, the normal PQPQ and the coordinate axes.

    Give your answer correct to 3 decimal places.

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Worked solution

14(a)

1. Differentiate the equation of the curve to find $\frac{dy}{dx}$.

y=−3(2x−4)−13y = -3(2x-4)^{-\frac{1}{3}}

dydx=−3(−13)(2x−4)−43⋅(2)\frac{dy}{dx} = -3 \left(-\frac{1}{3}\right)(2x-4)^{-\frac{4}{3}} \cdot (2)

dydx=2(2x−4)−43\frac{dy}{dx} = 2(2x-4)^{-\frac{4}{3}}

2. Find the gradient of the tangent at $P(-2, 1.5)$.

Substitute x=−2x = -2 into dydx\frac{dy}{dx}:
dydx=2(2(−2)−4)−43\frac{dy}{dx} = 2(2(-2)-4)^{-\frac{4}{3}}
dydx=2(−8)−43\frac{dy}{dx} = 2(-8)^{-\frac{4}{3}}
dydx=2(1(−8)43)=2(116)=18\frac{dy}{dx} = 2 \left(\frac{1}{(-8)^{\frac{4}{3}}}\right) = 2 \left(\frac{1}{16}\right) = \frac{1}{8}

3. Find the gradient of the normal at $P$.

Gradient of normal =−1Gradient of tangent= -\frac{1}{\text{Gradient of tangent}}
Gradient of normal =−118=−8= -\frac{1}{\frac{1}{8}} = -8

4. Find the equation of the normal and the coordinates of $Q$.

Equation of normal at P(−2,1.5)P(-2, 1.5):
y−1.5=−8(x−(−2))y - 1.5 = -8(x - (-2))
y−1.5=−8(x+2)y - 1.5 = -8(x + 2)

Since QQ lies on the x-axis, substitute y=0y = 0:
0−1.5=−8(x+2)0 - 1.5 = -8(x + 2)
−1.5=−8x−16-1.5 = -8x - 16
8x=−14.58x = -14.5
x=−14.58=−1.8125x = -\frac{14.5}{8} = -1.8125

Coordinates of QQ are (−1.8125,0)(-1.8125, 0).

Answer

(−1.8125,0)(-1.8125, 0)

14(b)

1. Find the gradient of the curve at $P(-2, 1.5)$ to determine the equation of the normal.

dydx=−3(−13)(2x−4)−4/3(2)=2(2x−4)−4/3\frac{dy}{dx} = -3 \left(-\frac{1}{3}\right)(2x-4)^{-4/3}(2) = 2(2x-4)^{-4/3}
At x=−2x = -2, dydx=2(−4−4)−4/3=2(−8)−4/3=2(1(−2)4)=216=18\frac{dy}{dx} = 2(-4-4)^{-4/3} = 2(-8)^{-4/3} = 2 \left(\frac{1}{(-2)^4}\right) = \frac{2}{16} = \frac{1}{8}

2. Find the coordinates of $Q$ (from part a).

Gradient of normal =−11/8=−8= -\frac{1}{1/8} = -8
Equation of normal at P(−2,1.5)P(-2, 1.5):
y−1.5=−8(x−(−2))y - 1.5 = -8(x - (-2))
y=−8x−16+1.5=−8x−14.5y = -8x - 16 + 1.5 = -8x - 14.5
At xx-axis, y=0y = 0:
0=−8x−14.5  ⟹  x=−14.58=−1.81250 = -8x - 14.5 \implies x = -\frac{14.5}{8} = -1.8125
So QQ is (−1.8125,0)(-1.8125, 0).

3. Calculate the area of the triangular region bounded by the normal, the line $x=-2$, and the $x$-axis.

Base of triangle =−1.8125−(−2)=0.1875= -1.8125 - (-2) = 0.1875
Height of triangle =1.5= 1.5
Area of triangle =12×0.1875×1.5=0.140625= \frac{1}{2} \times 0.1875 \times 1.5 = 0.140625

4. Calculate the area under the curve from $x = -2$ to $x = 0$.

∫−20−3(2x−4)−1/3dx=[−3(2x−4)2/323×2]−20\int_{-2}^{0} -3(2x-4)^{-1/3} dx = \left[ \frac{-3(2x-4)^{2/3}}{\frac{2}{3} \times 2} \right]_{-2}^{0}
=[−94(2x−4)2/3]−20= \left[ -\frac{9}{4}(2x-4)^{2/3} \right]_{-2}^{0}
=−94(−4)2/3−(−94(−8)2/3)= -\frac{9}{4}(-4)^{2/3} - \left( -\frac{9}{4}(-8)^{2/3} \right)
=−94(16)1/3+94(4)= -\frac{9}{4}(16)^{1/3} + \frac{9}{4}(4)
=9−94(24/3)≈3.33036= 9 - \frac{9}{4}(2^{4/3}) \approx 3.33036

5. Find the total shaded area.

Total Area =0.140625+3.33036=3.470985≈3.471= 0.140625 + 3.33036 = 3.470985 \approx 3.471 (to 3 d.p.)

Answer

3.4713.471

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