for your question, jetblack, you mentioned "find the equation of the tangent".
where is the tangent produced?
did you mean line joining P and Q?
If you did, then:
P is (c/rt2 , c/rt2)
Q is (-c/rt2 , c/rt2)
line joining, PQ, has the equation: y=c/rt2
thus, for the traingle OPQ, you have...