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June 2012 P11 Q5
377
The diagram shows the curve \(y = 7\sqrt{x}\) and the line \(y = 6x + k\), where \(k\) is a constant. The curve and the line intersect at the points \(A\) and \(B\).
For the case where \(k = 2\), find the \(x\)-coordinates of \(A\) and \(B\).
Solution
To find the \(x\)-coordinates of the intersection points, set the equations equal: \(7\sqrt{x} = 6x + 2\).
Rearrange to form a quadratic equation: \(6x + 2 = 7\sqrt{x} \Rightarrow 6(\sqrt{x})^2 - 7\sqrt{x} + 2 = 0\).
Let \(t = \sqrt{x}\), then the equation becomes \(6t^2 - 7t + 2 = 0\).
Factor the quadratic: \((3t - 2)(2t - 1) = 0\).
Solving for \(t\), we get \(t = \frac{2}{3}\) or \(t = \frac{1}{2}\).
Since \(t = \sqrt{x}\), we have \(\sqrt{x} = \frac{2}{3}\) or \(\sqrt{x} = \frac{1}{2}\).