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9.Straight Line
medium
In an isosceles triangle $ABC$, the coordinates of the points $B$ and $C$ on the base $BC$ are respectively $(1, 2)$ and $(2, 1)$. If the equation of the line $AB$ is $y = 2x$, then the equation of the line $AC$ is
A
$y = \frac{1}{2}(x - 1)$
B
$y = \frac{x}{2}$
C
$y = x - 1$
D
$2y = x + 3$
Solution

(b) $\angle ABC = \tan \theta = \frac{{\frac{1}{2} – 1}}{{1 + \frac{1}{2}}} = – \frac{1}{3}$ (Here ${m_1} = \frac{1}{2},\,{m_2} = 1)$
$AB = AC$; $\angle ABC = \angle ACB$
Hence $ – \frac{1}{3} = \frac{{m – 1}}{{1 + m}}$ ==> $m = \frac{1}{2}$
(Here m is the gradient of line $AC$)
Equation of line AC is,$y – 1 = \frac{1}{2}(x – 2)$ ==> $y = \frac{x}{2}$.
Standard 11
Mathematics