The table shows experimental values of two variables, x and y, which are connected by an equation of the form y=kxny = \frac{k}{x^{n}}, where n and k are constants.

  1. (a)

    Plot lg⁡y\lg y against lg⁡x\lg x and draw a straight line graph to illustrate this information.

  2. (b)

    Use your graph to estimate the value of kk and nn.

  3. (c)

    By drawing another straight line on the same axes, estimate the value of xx for which lg⁡y=lg⁡x2\lg y = \lg x^{2}.

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Worked solution

2(a)

1. Calculate the values of $\lg x$ and $\lg y$ for each data point, rounding to 2 decimal places.

For x=1.58x = 1.58, lg⁡x≈0.20\lg x \approx 0.20. For y=20.89y = 20.89, lg⁡y≈1.32\lg y \approx 1.32.
For x=2.00x = 2.00, lg⁡x≈0.30\lg x \approx 0.30. For y=15.14y = 15.14, lg⁡y≈1.18\lg y \approx 1.18.
For x=5.01x = 5.01, lg⁡x≈0.70\lg x \approx 0.70. For y=4.37y = 4.37, lg⁡y≈0.64\lg y \approx 0.64.
For x=13.80x = 13.80, lg⁡x≈1.14\lg x \approx 1.14. For y=1.10y = 1.10, lg⁡y≈0.04\lg y \approx 0.04.

lg⁡x\lg x0.200.300.701.14
lg⁡y\lg y1.321.180.640.04

2. Plot the points on a Cartesian plane and draw the line of best fit.

Choose appropriate scales for the axes (e.g., 2 cm2\text{ cm} to 0.20.2 units on both axes).
Plot the points (0.20,1.32)(0.20, 1.32), (0.30,1.18)(0.30, 1.18), (0.70,0.64)(0.70, 0.64), and (1.14,0.04)(1.14, 0.04).
Draw a straight line passing through or as close as possible to all the plotted points.

Answer

A straight line graph plotted with points (0.20,1.32)(0.20, 1.32), (0.30,1.18)(0.30, 1.18), (0.70,0.64)(0.70, 0.64), and (1.14,0.04)(1.14, 0.04).

2(b)

1. Transform the given non-linear equation into a linear form involving $\lg y$ and $\lg x$.

y=kxny = \frac{k}{x^n}
lg⁡y=lg⁡(kxn)\lg y = \lg \left(\frac{k}{x^n}\right)
lg⁡y=lg⁡k−lg⁡xn\lg y = \lg k - \lg x^n
lg⁡y=−nlg⁡x+lg⁡k\lg y = -n \lg x + \lg k

2. Calculate the gradient of the line to estimate $n$.

Using points from the table to find the gradient of the line of best fit (or using the theoretical points if the line passes through them perfectly):
Let's use (x1,y1)=(1.58,20.89)(x_1, y_1) = (1.58, 20.89) and (x2,y2)=(13.80,1.10)(x_2, y_2) = (13.80, 1.10).
lg⁡x1=lg⁡1.58≈0.1987\lg x_1 = \lg 1.58 \approx 0.1987
lg⁡y1=lg⁡20.89≈1.3199\lg y_1 = \lg 20.89 \approx 1.3199
lg⁡x2=lg⁡13.80≈1.1399\lg x_2 = \lg 13.80 \approx 1.1399
lg⁡y2=lg⁡1.10≈0.0414\lg y_2 = \lg 1.10 \approx 0.0414

Gradient=0.0414−1.31991.1399−0.1987=−1.27850.9412≈−1.358\text{Gradient} = \frac{0.0414 - 1.3199}{1.1399 - 0.1987} = \frac{-1.2785}{0.9412} \approx -1.358
Since gradient =−n= -n, n≈1.36n \approx 1.36 (accept 1.351.35 to 1.371.37).

3. Calculate the vertical intercept to estimate $k$.

Using the equation lg⁡y=−1.358lg⁡x+lg⁡k\lg y = -1.358 \lg x + \lg k and the point (0.1987,1.3199)(0.1987, 1.3199):
1.3199=−1.358(0.1987)+lg⁡k1.3199 = -1.358(0.1987) + \lg k
1.3199=−0.2698+lg⁡k1.3199 = -0.2698 + \lg k
lg⁡k=1.5897\lg k = 1.5897
k=101.5897≈38.9k = 10^{1.5897} \approx 38.9 (accept 38.038.0 to 40.040.0 depending on the line of best fit).

Answer

n=1.35n = 1.35, k=38.0k = 38.0

2(c)

1. Identify the equation of the straight line to be drawn

The given equation is lg⁡y=lg⁡x2\lg y = \lg x^{2}.
Using the power law of logarithms, this simplifies to lg⁡y=2lg⁡x\lg y = 2\lg x.
Letting Y=lg⁡yY = \lg y and X=lg⁡xX = \lg x, the line to draw is Y=2XY = 2X.

2. Find the intersection point and solve for x

Draw the line Y=2XY = 2X on the same axes.
From the graph, the intersection of this line with the line of best fit occurs at approximately X=0.473X = 0.473.
Since X=lg⁡xX = \lg x, we have lg⁡x=0.473\lg x = 0.473.
x=100.473≈2.97x = 10^{0.473} \approx 2.97

Answer

x≈2.97x \approx 2.97

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