The diagram shows part of the curve y=1(34x−5)2+1y = \frac{1}{\left(\frac{3}{4}x - 5\right)^2} + 1. PP is the point on the curve where x=6x = 6. The tangent to the curve at the point PP meets the xx-axis at QQ.

  1. (a)

    Find the coordinates of QQ.

  2. (b)

    Find the exact area of the shaded region bounded by the curve, the tangent PQPQ and the xx-coordinate axis.

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Worked solution

14(b)

1. Find the coordinates of $P$ and the gradient of the tangent at $P$.

When x=6x = 6,
y=1(34(6)−5)2+1=1(−0.5)2+1=5y = \frac{1}{\left(\frac{3}{4}(6) - 5\right)^2} + 1 = \frac{1}{(-0.5)^2} + 1 = 5
So PP is (6,5)(6, 5).

dydx=−2(34x−5)−3(34)=−32(34x−5)−3\frac{dy}{dx} = -2\left(\frac{3}{4}x - 5\right)^{-3} \left(\frac{3}{4}\right) = -\frac{3}{2}\left(\frac{3}{4}x - 5\right)^{-3}
At x=6x = 6, dydx=−32(−0.5)−3=12\frac{dy}{dx} = -\frac{3}{2}(-0.5)^{-3} = 12

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

Equation of tangent at PP:
y−5=12(x−6)y - 5 = 12(x - 6)
y=12x−67y = 12x - 67
At QQ, y=0  ⟹  12x=67  ⟹  x=6712y = 0 \implies 12x = 67 \implies x = \frac{67}{12}
So QQ is (6712,0)\left(\frac{67}{12}, 0\right).

3. Calculate the area under the curve from $x = 0$ to $x = 6$.

Area under curve =∫06((34x−5)−2+1)dx= \int_0^6 \left( \left(\frac{3}{4}x - 5\right)^{-2} + 1 \right) dx
=[(34x−5)−1−1⋅34+x]06= \left[ \frac{\left(\frac{3}{4}x - 5\right)^{-1}}{-1 \cdot \frac{3}{4}} + x \right]_0^6
=[−43(34x−5)+x]06= \left[ -\frac{4}{3\left(\frac{3}{4}x - 5\right)} + x \right]_0^6
=(−43(−0.5)+6)−(−43(−5)+0)= \left( -\frac{4}{3(-0.5)} + 6 \right) - \left( -\frac{4}{3(-5)} + 0 \right)
=(83+6)−(415)= \left( \frac{8}{3} + 6 \right) - \left( \frac{4}{15} \right)
=263−415=12615=425= \frac{26}{3} - \frac{4}{15} = \frac{126}{15} = \frac{42}{5}

4. Calculate the area of the triangle under the tangent and subtract it to find the shaded area.

The area under the tangent from x=6712x = \frac{67}{12} to x=6x = 6 forms a right-angled triangle.
Base of triangle =6−6712=512= 6 - \frac{67}{12} = \frac{5}{12}
Height of triangle =5= 5
Area of triangle =12×512×5=2524= \frac{1}{2} \times \frac{5}{12} \times 5 = \frac{25}{24}

Shaded Area =Area under curve−Area of triangle= \text{Area under curve} - \text{Area of triangle}
=425−2524=1008−125120=883120= \frac{42}{5} - \frac{25}{24} = \frac{1008 - 125}{120} = \frac{883}{120}

Answer

883120\frac{883}{120}

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