Question:

The point at which the circles $x^{2}+y^{2}-4 x-4 y+7=0 $ and $x^{2}+y^{2}-12 x -10 y+45=0$ touch each other, is

Updated On: Sep 5, 2023
  • $\left(\frac{13}{5}, \frac{14}{5}\right)$
  • $\left(\frac{2}{5}, \frac{5}{6}\right)$
  • $\left(\frac{14}{5}, \frac{13}{5}\right)$
  • $\left(\frac{12}{5}, 2+\frac{\sqrt{21}}{5}\right)$
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The Correct Option is C

Solution and Explanation

Centres and radii of given circles are $C_{1}(2,2), r_{1}=\sqrt{2^{2}+2^{2}}-7=1$ and $C_{2}(6,5)$,
$r_{2} =\sqrt{5^{2}+5^{2}-45} $ $=\sqrt{36+25-45}=4$ Let $P$ be the point at which the circle touch. Using internal ratio formula, $P(x, y)=\left(\frac{1 \times 6+4 \times 2}{1+4}, \frac{1 \times 5+4 \times 2}{1+4}\right)$ $=\left(\frac{6+8}{5}, \frac{5+8}{5}\right)=\left(\frac{14}{5}, \frac{13}{5}\right)$
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Concepts Used:

Straight lines

A straight line is a line having the shortest distance between two points. 

A straight line can be represented as an equation in various forms,  as show in the image below:

 

The following are the many forms of the equation of the line that are presented in straight line-

1. Slope – Point Form

Assume P0(x0, y0) is a fixed point on a non-vertical line L with m as its slope. If P (x, y) is an arbitrary point on L, then the point (x, y) lies on the line with slope m through the fixed point (x0, y0) if and only if its coordinates fulfil the equation below.

y – y0 = m (x – x0)

2. Two – Point Form

Let's look at the line. L crosses between two places. P1(x1, y1) and P2(x2, y2)  are general points on L, while P (x, y) is a general point on L. As a result, the three points P1, P2, and P are collinear, and it becomes

The slope of P2P = The slope of P1P2 , i.e.

\(\frac{y-y_1}{x-x_1} = \frac{y_2-y_1}{x_2-x_1}\)

Hence, the equation becomes:

y - y1 =\( \frac{y_2-y_1}{x_2-x_1} (x-x1)\)

3. Slope-Intercept Form

Assume that a line L with slope m intersects the y-axis at a distance c from the origin, and that the distance c is referred to as the line L's y-intercept. As a result, the coordinates of the spot on the y-axis where the line intersects are (0, c). As a result, the slope of the line L is m, and it passes through a fixed point (0, c). The equation of the line L thus obtained from the slope – point form is given by

y – c =m( x - 0 )

As a result, the point (x, y) on the line with slope m and y-intercept c lies on the line, if and only if

y = m x +c