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June 2016 p33 q8
2151
The points A and B have position vectors, relative to the origin O, given by \(\overrightarrow{OA} = \mathbf{i} + \mathbf{j} + \mathbf{k}\) and \(\overrightarrow{OB} = 2\mathbf{i} + 3\mathbf{k}\). The line \(l\) has vector equation \(\mathbf{r} = 2\mathbf{i} - 2\mathbf{j} - \mathbf{k} + \mu(-\mathbf{i} + 2\mathbf{j} + \mathbf{k})\).
(i) Show that the line passing through A and B does not intersect \(l\).
(ii) Show that the length of the perpendicular from A to \(l\) is \(\frac{1}{\sqrt{2}}\).
Solution
(i) The vector equation of the line through A and B is \(\mathbf{r} = \mathbf{i} + \mathbf{j} + \mathbf{k} + \lambda(\mathbf{i} - \mathbf{j} + 2\mathbf{k})\).
Equating components with the line \(l\):
\(1 + \lambda = 2 - \mu\)
\(1 - \lambda = -2 + 2\mu\)
\(1 + 2\lambda = -1 + \mu\)
Solving these equations, we find \(\lambda = -1\) and \(\mu = 2\), but the third equation is not satisfied, showing no intersection.
(ii) To find the perpendicular distance from A to \(l\), consider a point \(P\) on \(l\) given by \((1-\mu)\mathbf{i} + (-3 + 2\mu)\mathbf{j} + (-2 + \mu)\mathbf{k}\).
Calculate \(\overrightarrow{AP}\) and find \(\mu\) such that \(\overrightarrow{AP}\) is perpendicular to the direction vector of \(l\), \(-\mathbf{i} + 2\mathbf{j} + \mathbf{k}\).
Solving gives \(\mu = \frac{3}{2}\).
Substitute \(\mu = \frac{3}{2}\) into \(\overrightarrow{AP}\) and find its magnitude: \(\frac{1}{\sqrt{2}}\).