The diagram shows part of the curve y=15(0.5x+1)y = \frac{15}{(0.5x+1)} and a straight line y=1.25x−10y = 1.25x - 10. The straight line intersects the yy-axis at PP, the xx-axis at QQ and the curve at RR.

  1. (a)

    Find the coordinates of PP and QQ.

  2. (b)

    Find the coordinates of RR.

  3. (c)

    Find the area of the shaded region bounded by the curve, the line y=1.25x−10y = 1.25x - 10 and the yy-axis.

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Worked solution

10(a)

1. Find the coordinates of $P$ by setting $x = 0$

For the yy-intercept PP, substitute x=0x = 0 into the equation of the line:
y=1.25(0)−10y = 1.25(0) - 10
y=−10y = -10
So, PP is (0,−10)(0, -10).

2. Find the coordinates of $Q$ by setting $y = 0$

For the xx-intercept QQ, substitute y=0y = 0 into the equation of the line:
0=1.25x−100 = 1.25x - 10
1.25x=101.25x = 10
x=101.25=8x = \frac{10}{1.25} = 8
So, QQ is (8,0)(8, 0).

Answer

P(0,−10)P(0, -10), Q(8,0)Q(8, 0)

10(b)

1. Equate the equations of the curve and the line to find the intersection points

150.5x+1=1.25x−10\frac{15}{0.5x+1} = 1.25x - 10

2. Rearrange into a standard quadratic equation

15=(1.25x−10)(0.5x+1)15 = (1.25x - 10)(0.5x + 1)
15=0.625x2+1.25x−5x−1015 = 0.625x^2 + 1.25x - 5x - 10
0.625x2−3.75x−25=00.625x^2 - 3.75x - 25 = 0
Multiply by 8 to remove decimals:
5x2−30x−200=05x^2 - 30x - 200 = 0
x2−6x−40=0x^2 - 6x - 40 = 0

3. Solve the quadratic equation for $x$

(x−10)(x+4)=0(x - 10)(x + 4) = 0
x=10 or x=−4x = 10 \text{ or } x = -4

4. Determine the valid coordinates of $R$

From the diagram, RR is in the first quadrant, so x>0x > 0. Therefore, x=10x = 10.
Substitute x=10x = 10 into the equation of the line:
y=1.25(10)−10=12.5−10=2.5y = 1.25(10) - 10 = 12.5 - 10 = 2.5
The coordinates of RR are (10,2.5)(10, 2.5).

Answer

(10,2.5)(10, 2.5)

10(c)

1. Find the $x$-coordinate of the intersection point $R$ by equating the equations of the curve and the line.

150.5x+1=1.25x−10\frac{15}{0.5x+1} = 1.25x - 10
15=(1.25x−10)(0.5x+1)15 = (1.25x - 10)(0.5x + 1)
15=0.625x2+1.25x−5x−1015 = 0.625x^2 + 1.25x - 5x - 10
0.625x2−3.75x−25=00.625x^2 - 3.75x - 25 = 0
Multiply by 8:
5x2−30x−200=05x^2 - 30x - 200 = 0
x2−6x−40=0x^2 - 6x - 40 = 0
(x−10)(x+4)=0(x - 10)(x + 4) = 0
Since RR is in the first quadrant, x=10x = 10.

2. Set up the definite integral for the area bounded by the curve, the line, and the $y$-axis.

Area=∫010(150.5x+1−(1.25x−10))dx\text{Area} = \int_{0}^{10} \left( \frac{15}{0.5x+1} - (1.25x - 10) \right) dx

3. Integrate the expression.

∫(150.5x+1−1.25x+10)dx=[15ln⁡(0.5x+1)0.5−1.25x22+10x]010\int \left( \frac{15}{0.5x+1} - 1.25x + 10 \right) dx = \left[ \frac{15 \ln(0.5x+1)}{0.5} - \frac{1.25x^2}{2} + 10x \right]_0^{10}
=[30ln⁡(0.5x+1)−0.625x2+10x]010= \left[ 30 \ln(0.5x+1) - 0.625x^2 + 10x \right]_0^{10}

4. Evaluate the definite integral to find the area.

Substitute x=10x = 10:
30ln⁡(0.5(10)+1)−0.625(10)2+10(10)=30ln⁡(6)−62.5+100=30ln⁡6+37.530 \ln(0.5(10)+1) - 0.625(10)^2 + 10(10) = 30 \ln(6) - 62.5 + 100 = 30 \ln 6 + 37.5
Substitute x=0x = 0:
30ln⁡(1)−0+0=030 \ln(1) - 0 + 0 = 0
Area=30ln⁡6+37.5≈91.2527≈91.3 units2\text{Area} = 30 \ln 6 + 37.5 \approx 91.2527 \approx 91.3 \text{ units}^2

Answer

30ln⁡6+37.530 \ln 6 + 37.5 or 91.391.3 units2^2

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