9.Straight Line
normal

If one vertex of an equilateral triangle of side $'a'$ lies at the origin and the other lies on the line $x - \sqrt{3} y = 0$ then the co-ordinates of the third vertex are :

A

$(0, a)$

B

$\left( {\frac{{\sqrt 3 \,a}}{2}\,\,,\,\, - \,\frac{a}{2}} \right)$

C

$(0, - a)$

D

all of the above

Solution

make a figure and interpret 

Slope of the line $x=\sqrt{3} y$ is $\frac{1}{\sqrt{3}}$ which is $30^{\circ}$ with the positive $x$ -axis. As it is an equilateral triangle the other vertex must be on $x=-\sqrt{3} y$

Let the side be $(x, y)$. Hence, $x^{2}+y^{2}=a^{2}$

$4 y^{2}=a^{2}$

$y=\pm \frac{a}{2}$

similarily, $x=\pm \frac{\sqrt{3} a}{2}$

Hence, if the triangle lies in the 1 st quadrant and 4 th quadrant vertex is $\left(\frac{\sqrt{3} a}{2},-\frac{a}{2}\right)$

If the triangle lies in the 2 nd and 3 rd quadrant the vertex is $\left(-\frac{\sqrt{3} a}{2},-\frac{a}{2}\right)$

Standard 11
Mathematics

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