MCQ
The foot of the perpendicular drawn from the origin, on the line, $3x + y = \lambda \,\left( {\lambda  \ne 0} \right)$ is $P$. If the line meets $x-$ axis at $A$ and $y-$ axis at $B$, then the ratio $BP : PA$ is
  • A
    $9: 1$
  • B
    $1 : 3$
  • C
    $1 : 9$
  • $3 : 1$

Answer

Correct option: D.
$3 : 1$
d
Equation of the lien, which is perpendicular to the line, $3x + y = \lambda \,\left( {\lambda  \ne 0} \right)$ and passing through origin, is given by

$\frac{{x - 0}}{3} = \frac{{y - 0}}{1} = r$

For foot of perpendicular

$r = \frac{{ - \left( {\left( {3 \times 0} \right) + \left( {1 \times 0} \right) - \lambda } \right)}}{{{3^2} + {1^2}}} = \frac{\lambda }{{10}}$

So, food of perpendicular $P = \left( {\frac{{3\lambda }}{{10}},\frac{\lambda }{{10}}} \right)$

Given the line meets $X$-asix at $A = \left( {\frac{\lambda }{3},0} \right)$

and meets $Y$-axis at $B = \left( {o,\lambda } \right)$

So,

$BP = \sqrt {{{\left( {\frac{{3\lambda }}{{10}}} \right)}^2} + {{\left( {\frac{\lambda }{{10}} - \lambda } \right)}^2}}  \Rightarrow BP = \sqrt {\frac{{9{\lambda ^2}}}{{100}} + \frac{{81{\lambda ^2}}}{{100}}} $

$ \Rightarrow BP = \sqrt {\frac{{90{\lambda ^2}}}{{100}}} $

Now,$PA = \sqrt {{{\left( {\frac{\lambda }{3} - \frac{{3\lambda }}{{10}}} \right)}^2} + {{\left( {0 + \frac{\lambda }{{10}}} \right)}^2}} $

 $PA = \sqrt {\frac{{{\lambda ^2}}}{{900}} + \frac{{{\lambda ^2}}}{{100}}}  \Rightarrow PA = \sqrt {\frac{{10{\lambda ^2}}}{{900}}} $

Therefore $BP:PA = 3:1$

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