Ellipse 2 Question 15

15. If a>2b>0, then positive value of m for which y=mxb1+m2 is a common tangent to x2+y2=b2 and (xa)2+y2=b2 is

(2002,1 M)

(a) 2ba24b2

(b) a24b22b

(c) 2ba2b

(d) ba2b

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Answer:

Correct Answer: 15. (a)

Solution:

  1. Given, y=mxb1+m2 touches both the circles, so distance from centre = radius of both the circles.

|ma0b1+m2|m2+1=b and |b1+m2|m2+1=b|mab1+m2|=|b1+m2|m2a22abm1+m2+b2(1+m2)=b2(1+m2)ma2b1+m2=0m2a2=4b2(1+m2)m=2ba24b2



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