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Feb/Mar 2018 p12 q4
570
A straight line cuts the positive x-axis at A and the positive y-axis at B (0, 2). Angle BAO = \(\frac{1}{6} \pi\) radians, where O is the origin.
(i) Find the exact value of the x-coordinate of A.
(ii) Find the equation of the perpendicular bisector of AB, giving your answer in the form \(y = mx + c\), where \(m\) is given exactly and \(c\) is an integer.
Solution
(i) The line cuts the x-axis at A, so the y-coordinate of A is 0. The angle \(\angle BAO = \frac{1}{6} \pi\) radians implies that the slope of the line is \(\tan\left(\frac{1}{6} \pi\right) = \frac{1}{\sqrt{3}}\).
The equation of the line is \(y - 2 = \frac{1}{\sqrt{3}}(x - 0)\).
Setting \(y = 0\) to find the x-coordinate of A:
\(0 - 2 = \frac{1}{\sqrt{3}}x\)
\(x = 2\sqrt{3}\)
(ii) The midpoint of AB is \(\left(\frac{\sqrt{3}}{2}, 1\right)\).
The gradient of AB is \(-\frac{1}{\sqrt{3}}\), so the gradient of the perpendicular bisector is \(\sqrt{3}\).
The equation of the perpendicular bisector is \(y - 1 = \sqrt{3}(x - \frac{\sqrt{3}}{2})\).