June 2010 p33 q2
1572
The variables x and y satisfy the equation y3 = Ae2x, where A is a constant. The graph of ln y against x is a straight line.
(i) Find the gradient of this line.
(ii) Given that the line intersects the axis of ln y at the point where ln y = 0.5, find the value of A correct to 2 decimal places.
Solution
To solve part (i), start with the given equation:
\(y^3 = Ae^{2x}\)
Take the natural logarithm of both sides:
\(\ln(y^3) = \ln(Ae^{2x})\)
Using logarithm properties, this becomes:
\(3\ln(y) = \ln(A) + 2x\)
This is in the form \(y = mx + c\), where the gradient \(m\) is \(\frac{2}{3}\).
For part (ii), substitute \(x = 0\) and \(\ln(y) = 0.5\) into the equation:
\(3(0.5) = \ln(A) + 2(0)\)
\(1.5 = \ln(A)\)
Exponentiate both sides to solve for \(A\):
\(A = e^{1.5}\)
Calculate \(A\) to two decimal places:
\(A \approx 4.48\)
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