May 5th, 2009, 03:01 AM  #1 
Newbie Joined: Apr 2009 Posts: 28 Thanks: 0  Tangent Problem If the line px + qy = r is a tangent to the ellipse a^2.x^2 + b^2.y^2 = c^2, then the maximum possible value of pq is (A) max{a, b}·r/c (B) a^2b^2r^2/2c^4 (C) abr^2/c^2 (D) abr^2/2c^2. Ans: lx + my + n = 0 is a tangent of the ellipse x^2/a^2 + y^2/b^2 = 1 if n^2 = a^2.l^2 + b^2.m^2 .....(I) Here the line is px + qy = r. Eqation of the ellipse a^2x^2 + b^2y^2 = c^2 or, x^2/(c^2/a^2) + y^2/(c^2/b^2) = 1 So from (I), r^2 = (c^2/a^2).p^2 + (c^2/b^2).q^2 How the maximum possible value of pqwill be found from above equation? Thanks in advance!! 

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