The table shows experimental values of two variables, xx and yy.
xx2468
yy2.250.810.470.33

  1. (i)

    On the graph paper, plot xyxy against 1x\frac{1}{x} and draw a straight line graph.

  2. (ii)

    Using your graph, estimate the values of xx and yy for which x=3yx = \frac{3}{y}.

  3. (iii)

    express yy in terms of xx.

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Worked solution

11(i)

1. Calculate the values of $X = \frac{1}{x}$ and $Y = xy$ for each data point.

For x=2,y=2.25x=2, y=2.25: X=12=0.5X = \frac{1}{2} = 0.5, Y=2(2.25)=4.5Y = 2(2.25) = 4.5
For x=4,y=0.81x=4, y=0.81: X=14=0.25X = \frac{1}{4} = 0.25, Y=4(0.81)=3.24Y = 4(0.81) = 3.24
For x=6,y=0.47x=6, y=0.47: X=16≈0.167X = \frac{1}{6} \approx 0.167, Y=6(0.47)=2.82Y = 6(0.47) = 2.82
For x=8,y=0.33x=8, y=0.33: X=18=0.125X = \frac{1}{8} = 0.125, Y=8(0.33)=2.64Y = 8(0.33) = 2.64

2. Plot the points $(X, Y)$ on the graph paper and draw the line of best fit.

Plot the points (0.5,4.5)(0.5, 4.5), (0.25,3.24)(0.25, 3.24), (0.167,2.82)(0.167, 2.82), and (0.125,2.64)(0.125, 2.64) on a grid with XX-axis representing 1x\frac{1}{x} and YY-axis representing xyxy. Draw a straight line passing through or close to these points.

Answer

Graph plotted with points (0.5,4.5)(0.5, 4.5), (0.25,3.24)(0.25, 3.24), (0.167,2.82)(0.167, 2.82), (0.125,2.64)(0.125, 2.64) and a straight line drawn through them.

11(ii)

1. Rearrange the given equation to match the vertical axis variable of the graph.

The given equation is x=3yx = \frac{3}{y}.
Rearranging this gives xy=3xy = 3.

2. Use the graph to find the corresponding value of the horizontal axis variable.

On the graph of xyxy against 1x\frac{1}{x}, locate the point where the vertical coordinate is xy=3xy = 3.
Reading the corresponding horizontal coordinate from the straight line graph gives:
1x≈0.198\frac{1}{x} \approx 0.198

3. Calculate the values of $x$ and $y$.

x=10.198≈5.05x = \frac{1}{0.198} \approx 5.05
Substitute xx back into the equation to find yy:
y=35.05≈0.594y = \frac{3}{5.05} \approx 0.594
(Note: Acceptable range depends on the student's plotted graph, typically x≈5.0x \approx 5.0 to 5.15.1 and y≈0.59y \approx 0.59 to 0.600.60)

Answer

x≈5.06x \approx 5.06, y≈0.593y \approx 0.593

11(iii)

1. Determine the gradient and $Y$-intercept of the straight line graph of $xy$ against $\frac{1}{x}$

From the table, the coordinates (X,Y)=(1x,xy)(X, Y) = (\frac{1}{x}, xy) are:
x=2  ⟹  X=0.5,Y=4.5x=2 \implies X=0.5, Y=4.5
x=4  ⟹  X=0.25,Y=3.24x=4 \implies X=0.25, Y=3.24
x=6  ⟹  X=0.167,Y=2.82x=6 \implies X=0.167, Y=2.82
x=8  ⟹  X=0.125,Y=2.64x=8 \implies X=0.125, Y=2.64

Gradient m=4.5−3.240.5−0.25=1.260.25=5.04≈5m = \frac{4.5 - 3.24}{0.5 - 0.25} = \frac{1.26}{0.25} = 5.04 \approx 5 (or using points (0.5,4.5)(0.5, 4.5) and (0.125,2.64)(0.125, 2.64), m=4.5−2.640.5−0.125=1.860.375=4.96≈5m = \frac{4.5 - 2.64}{0.5 - 0.125} = \frac{1.86}{0.375} = 4.96 \approx 5).
Let's assume the exact gradient is 55.
YY-intercept cc: 4.5=5(0.5)+c  ⟹  c=24.5 = 5(0.5) + c \implies c = 2.

2. Form the linear equation relating $Y$ and $X$

The equation of the line is Y=mX+cY = mX + c.
Substituting m=5m = 5 and c=2c = 2, we get:
Y=5X+2Y = 5X + 2

3. Convert the linear equation back to the original variables $x$ and $y$

Substitute Y=xyY = xy and X=1xX = \frac{1}{x}:
xy=5(1x)+2xy = 5\left(\frac{1}{x}\right) + 2
xy=5x+2xy = \frac{5}{x} + 2
Divide by xx:
y=5x2+2xy = \frac{5}{x^2} + \frac{2}{x}

Answer

y=2x+1x2y = \frac{2}{x} + \frac{1}{x^2}

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