The diagram shows part of the curve y=52x−1y = \frac{5}{2x - 1}. The line 9y=−10x+559y = -10x + 55 intersects the curve at points AA and BB and meets the xx-axis at point CC. CDCD is a straight line parallel to the yy-axis.

  1. (i)

    Find the coordinates of AA, BB and CC.

  2. (ii)

    Find the area of the shaded region.

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Worked solution

10(ii)

1. Find the coordinates of A, B, and C (from part i) to determine the limits of integration.

From part (i), solving y=52x−1y = \frac{5}{2x - 1} and 9y=−10x+559y = -10x + 55 simultaneously gives x=1x = 1 and x=5x = 5.
AA is (1,5)(1, 5) and BB is (5,59)(5, \frac{5}{9}).
The line meets the xx-axis at CC, so −10x+55=0  ⟹  x=5.5-10x + 55 = 0 \implies x = 5.5.
CC is (5.5,0)(5.5, 0).

2. Set up the integrals for the two shaded regions.

The shaded region is split into two parts at x=5x = 5 (point B).
For 1⩽x⩽51 \leqslant x \leqslant 5, the line is above the curve.
For 5⩽x⩽5.55 \leqslant x \leqslant 5.5, the curve is above the line.
Total Area = ∫15(−10x+559−52x−1)dx+∫55.5(52x−1−−10x+559)dx\int_{1}^{5} \left( \frac{-10x + 55}{9} - \frac{5}{2x - 1} \right) dx + \int_{5}^{5.5} \left( \frac{5}{2x - 1} - \frac{-10x + 55}{9} \right) dx

3. Integrate the functions.

∫−10x+559dx=−5x2+55x9\int \frac{-10x + 55}{9} dx = \frac{-5x^2 + 55x}{9}
∫52x−1dx=52ln⁡(2x−1)\int \frac{5}{2x - 1} dx = \frac{5}{2} \ln(2x - 1)

4. Evaluate the definite integrals to find the total area.

Area 1 (from x=1x=1 to x=5x=5):
Area under line = [−5(25)+55(5)9]−[−5(1)+55(1)9]=1509−509=1009\left[ \frac{-5(25) + 55(5)}{9} \right] - \left[ \frac{-5(1) + 55(1)}{9} \right] = \frac{150}{9} - \frac{50}{9} = \frac{100}{9}
Area under curve = [52ln⁡(9)]−[52ln⁡(1)]=52ln⁡9=5ln⁡3\left[ \frac{5}{2} \ln(9) \right] - \left[ \frac{5}{2} \ln(1) \right] = \frac{5}{2} \ln 9 = 5 \ln 3
Area 1 = 1009−5ln⁡3≈5.6181\frac{100}{9} - 5 \ln 3 \approx 5.6181

Area 2 (from x=5x=5 to x=5.5x=5.5):
Area under curve = [52ln⁡(10)]−[52ln⁡(9)]=52ln⁡(109)\left[ \frac{5}{2} \ln(10) \right] - \left[ \frac{5}{2} \ln(9) \right] = \frac{5}{2} \ln\left(\frac{10}{9}\right)
Area under line = [−5(5.5)2+55(5.5)9]−1509=151.259−1509=1.259=536\left[ \frac{-5(5.5)^2 + 55(5.5)}{9} \right] - \frac{150}{9} = \frac{151.25}{9} - \frac{150}{9} = \frac{1.25}{9} = \frac{5}{36}
Area 2 = 52ln⁡(109)−536≈0.1245\frac{5}{2} \ln\left(\frac{10}{9}\right) - \frac{5}{36} \approx 0.1245

Total Area = 5.6181+0.1245=5.7426≈5.745.6181 + 0.1245 = 5.7426 \approx 5.74 (to 3 s.f.)

Answer

5.745.74 (to 3 s.f.)

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