If a circle, whose centre is $(-1, 1)$ touches the straight line $x + 2y + 12 = 0$, then the coordinates of the point of contact are
A$\left(\frac{-7}{2}, -4\right)$
B$\left(\frac{-18}{5}, \frac{-21}{5}\right)$
C$(2, -7)$
D$(-2, -5)$
Answer & Solution
Correct answer: B. $\left(\frac{-18}{5}, \frac{-21}{5}\right)$
The point of contact is the foot of the perpendicular from the centre $(-1,1)$ to the line $x+2y+12=0$.
For the line $ax+by+c=0$, the foot of the perpendicular from $(x_1,y_1)$ is
$$\left(x_1-\frac{a(ax_1+by_1+c)}{a^2+b^2},\ y_1-\frac{b(ax_1+by_1+c)}{a^2+b^2}\right).$$
Here $a=1$, $b=2$, $c=12$, $(x_1,y_1)=(-1,1)$.
$$ax_1+by_1+c=1\cdot(-1)+2\cdot1+12=13$$
$$a^2+b^2=1^2+2^2=5$$
So the foot of the perpendicular is
$$\left(-1-\frac{13}{5},\ 1-\frac{26}{5}\right)$$
$$\left(\frac{-18}{5},\frac{-21}{5}\right).$$
Now checking the options, this matches option $\mathrm{B}$.
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