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Nov 2005 p3 q2
1574
Two variable quantities x and y are related by the equation \(y = Ax^n\), where A and n are constants. The diagram shows the result of plotting \(\\ln y\) against \(\\ln x\) for four pairs of values of x and y. Use the diagram to estimate the values of A and n.
Solution
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The given equation is \(y = Ax^n\). Taking natural logarithms on both sides, we get:
\(\\ln y = \\ln A + n \\ln x\)
This is in the form of a straight line \(y = mx + c\), where \(m = n\) and \(c = \\ln A\).
From the graph, estimate the y-intercept (\(\\ln A\)) and the gradient (\(n\)).
Estimate the y-intercept from the graph, which is approximately 0.7. Therefore, \(\\ln A = 0.7\), giving \(A = e^{0.7} \approx 2.0\).
Calculate the gradient using two points from the graph. For example, using points (0.5, 0.5) and (2.5, 1.0):