JEE MAIN - Mathematics (2018 (Offline) - No. 11)
A straight line through a fixed point (2, 3) intersects the coordinate axes at distinct points P and Q. If O is
the origin and the rectangle OPRQ is completed, then the locus of R is :
3x + 2y = 6xy
3x + 2y = 6
2x + 3y = xy
3x + 2y = xy
Explanation
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Let coordinate of point R = (h, k).
Equation of line PQ,
(y $$-$$ 3) = m (x $$-$$ 2).
Put y = 0 to get coordinate of point p,
0 $$-$$ 3 = (x $$-$$ 2)
$$ \Rightarrow $$ x = 2 $$-$$ $${3 \over m}$$
$$\therefore\,\,\,$$ p = (2 $$-$$ $${3 \over m}$$, 0)
As p = (h, 0) then
h = 2 $$-$$ $${3 \over m}$$
$$ \Rightarrow $$ $${3 \over m}$$ = 2 $$-$$ h
$$ \Rightarrow $$ m = $${3 \over {2 - h}}$$ . . . . . . (1)
Put x = 0 to get coordinate of point Q,
y $$-$$ 3 $$=$$ m (0 $$-$$ 2)
$$ \Rightarrow $$ y = 3 $$-$$ 2m
$$\therefore\,\,\,$$ point Q = (0, 3 $$-$$ 2m)
And From the graph you can see Q = (0, k).
$$\therefore\,\,\,$$ k = 3 $$-$$ 2m
$$ \Rightarrow $$ m = $${{3 - k} \over 2}$$ . . . . (2)
By comparing (1) and (2) get
$${3 \over {2 - h}} = {{3 - k} \over 2}$$
$$ \Rightarrow $$ (2 $$-$$ h)(3 $$-$$ k) = 6
$$ \Rightarrow $$ 6 $$-$$ 3h $$-$$ 2K + hk = 6
$$ \Rightarrow $$ 3 h + 2K = hk
$$\therefore\,\,\,$$ locus of point R is 3x + 2y= xy
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