MCQ
Let $k$ be an integer such that triangle with vertices $\left( {k, - 3k} \right),\left( {5,k} \right)$ and $\left( { - k,2} \right)$ has area $28$ sq. units. Then the orthocentre of his triangle is at the point :
  • $\left( {2,\frac{1}{2}\;} \right)\;$
  • B
    $\left( {2,\frac{{ - 1}}{2}\;} \right)$
  • C
    $\left( {1,\frac{3}{4}\;} \right)$
  • D
    $\left( {1,\frac{{ - 3}}{4}\;} \right)$

Answer

Correct option: A.
$\left( {2,\frac{1}{2}\;} \right)\;$
a
(1)   We  have 

$\frac{1}{2}\,\left\| \begin{array}{l}
\,k\,\,\, - 3k\,\,\,1\\
\,5\,\,\,\,\,\,\,\,k\,\,\,\,\,1\\
 - k\,\,\,\,\,2\,\,\,\,\,1
\end{array} \right\| = 28$

$ \Rightarrow 5{k^2} + 13k - 46 = 0$

or $5{k^2} + 13k - 66 = 0$

Now, $5{k^2} + 13k - 46 = 0$

$ \Rightarrow k = \frac{{ - 13 \pm \sqrt {1089} }}{{10}}$

$\therefore k = \frac{{ - 23}}{5};k = 2$

since $k$ is an integer, $\therefore k = 2$

Also $5{k^2} + 13k + 66 = 0$

$ \Rightarrow k = \frac{{ - 13 \pm \sqrt { - 1151} }}{{10}}$

So no real solution exist 

For cothocentre

$BH \bot AC$

$\therefore \left( {\frac{{\beta  - 2}}{{\alpha  - 5}}} \right)\left( {\frac{8}{{ - 4}}} \right) =  - 1$

$ \Rightarrow \,\,\alpha  - 2\beta  = 1\,\,\,\,\,\,\,\,\,\,.......\left( 1 \right)$

Also $CH \bot AB$

$\therefore \left( {\frac{{\beta  - 2}}{{\alpha  + 2}}} \right)\left( {\frac{8}{3}} \right) =  - 1$

$ \Rightarrow \,\,3\alpha  - 8\beta  = 1\,0\,\,\,\,\,\,\,\,\,.......\left( 2 \right)$

Solving $(1)$ and $(2)$ , we get

$\alpha  = 2,\beta  = \frac{1}{2}$

orthocentre is $\left( {2,\frac{1}{2}} \right)$

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