Any circle passing through the points of intersection of the given line and circle has the equation $${x^2} + {y^2} - 9 + \lambda \left( {x + y - 1} \right) = 0.$$ Its centre $$ = \left( { - \frac{\lambda }{2},\, - \frac{\lambda }{2}} \right).$$
The circle is the smallest if $$\left( { - \frac{\lambda }{2},\, - \frac{\lambda }{2}} \right)$$ is on the chord $$x + y = 1.$$
$$\therefore \,\, - \frac{\lambda }{2} - \frac{\lambda }{2} = 1\,\,\,\,\, \Rightarrow \lambda = - 1$$
Putting this value for $$\lambda ,$$ the equation of the smallest circle is $${x^2} + {y^2} - 9 - \left( {x + y - 1} \right) = 0.$$
122.
The equation of the circle of radius $$2\sqrt 2 $$ whose centre lies on the line $$x - y = 0$$ and which touches the line $$x + y = 4,$$ and whose centre’s coordinates satisfy the inequality $$x + y > 4$$ is :
123.
The equations of two circles are $${x^2} + {y^2} + 2\lambda x + 5 = 0$$ and $${x^2} + {y^2} + 2\lambda y + 5 = 0.\,P$$ is any point on the line $$x - y = 0. $$ If $$PA$$ and $$PB$$ are the lengths of the tangents from $$P$$ to the two circles and $$PA = 3$$ then $$PB$$ is equal to :
The centre of two circles are $${C_1}\left( {2,\,2} \right)$$ and $${C_2}\left( {0,\,0} \right).$$ The radii of two circles are $${r_1} = 2\sqrt 2 $$ and $${r_2} = 4$$
The equation of the common chord of the circles $${x^2} + {y^2} - 4x - 4y = 0$$ and $${x^2} + {y^2} = 16$$ is $$x + y = 4$$ which meets the circle $${x^2} + {y^2} = 16$$ at points $$A\left( {4,\,0} \right)$$ and $$B\left( {0,\,4} \right).$$ Obviously $$OA \bot OB.$$ Hence, the common chord $$AB$$ makes a right angle at the centre of the circle $${x^2} + {y^2} = 16.$$ Where, $$O$$ is the origin and the centre $${C_2}$$ of the second circle.
125.
If the pair of lines $$a{x^2} + 2\left( {a + b} \right)xy + b{y^2} = 0$$ lie along diameters of a circle and divide the circle into four sectors such that the area of one of the sectors is thrice the area of another sector then :
The length of the perpendicular from the centre (0, 0) to the line $$ = \frac{2}{{\sqrt {1 + {m^2}} }}$$
The radius of the circle $$= 1.$$ For the line to cut the circle at distinct or
coincident points, $$\frac{2}{{\sqrt {1 + {m^2}} }} \leqslant 1{\text{ or }}1 + {m^2} \geqslant 4{\text{ or }}{m^2} \geqslant 3.$$
129.
The length of the diameter of the circle which touches the $$x$$-axis at the point $$\left( {1,\,0} \right)$$ and passes through the point $$\left( {2,\,3} \right)$$ is:
Let centre of the circle be $$\left( {1,\,h} \right)$$ [ $$\because $$ circle touches $$x$$-axis at $$\left( {1,\,0} \right)$$ ]
Let the circle passes through the point $$B\left( {2,\,3} \right)$$
$$\eqalign{
& \therefore CA = CB\,\,\,\,\,\,\,\left( {{\text{radius}}} \right) \cr
& \Rightarrow C{A^2} = C{B^2} \cr
& \Rightarrow {\left( {1 - 1} \right)^2} + {\left( {h - 0} \right)^2} = {\left( {1 - 2} \right)^2} + {\left( {h - 3} \right)^2} \cr
& \Rightarrow {h^2} = 1 + {h^2} + 9 - 6h \cr
& \Rightarrow h = \frac{{10}}{6} = \frac{5}{3} \cr} $$
Thus, diameter is $$2h = \frac{{10}}{3}$$
130.
The circle passing through $$\left( {1,\, - 2} \right)$$ and touching the axis of $$x$$ at $$\left( {3,\,0} \right)$$ also passes through the point -