Question
If the plane $$2ax - 3ay + 4az + 6 = 0$$ passes through the midpoint of the line joining the centres of the spheres $${x^2} + {y^2} + {z^2} + 6x - 8y - 2z = 13$$ and $${x^2} + {y^2} + {z^2} - 10x + 4y - 2z = 8$$ then $$a$$ equals :
A.
$$ - 1$$
B.
$$1$$
C.
$$ - 2$$
D.
$$2$$
Answer :
$$ - 2$$
Solution :
Plane $$2ax - 3ay + 4az + 6 = 0$$ passes through the mid point of the centre of spheres $${x^2} + {y^2} + {z^2} + 6x - 8y - 2z = 13$$ and $${x^2} + {y^2} + {z^2} - 10x + 4y - 2z = 8$$ respectively.
Center of spheres are $$\left( { - 3,\,4,\,1} \right)$$ and $$\left( {5,\, - 2,\,1} \right).$$
Mid point of centres is $$\left( {1,\,1,\,1} \right).$$
Satisfying this in the equation of plane, we get
$$2a - 3a + 4a + 6 = 0 \Rightarrow a = - 2.$$