IIT JEE , AIEEE Forum - Mathematics , Analytical Geometry

rik_mad

For the second question..what u need to know is how did we cum up with the condition of tangency formula
y = mx +c
circle is
x^2 + y^2 = a^2
now u substitute y from line eqn in cirlce equation to get
x^2 + m^2x^2 + c^2 + 2mcx = a^2
now a tangent is only possible when the above equation is satisfied by only one value of x . so we check for D to be 0..and thus we end up wit formula..
for general circle eqn u need to apply the same procedue. the same formula cannot be used as it is inly valid for circles with centre as origin.

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