Ellipse 2 Question 23

23. Suppose that the foci of the ellipse x29+y25=1 are (f1,0) and (f2,0), where f1>0 and f2<0. Let P1 and P2 be two parabolas with a common vertex at (0,0) with foci at (f1,0) and (2f2,0), respectively. Let T1 be a tangent to P1 which passes through (2f2,0) and T2 be a tangent to P2 which passes through (f1,0). If m1 is the slope of T1 and m2 is the slope of T2, then the value of 1m12+m22 is

(2015 Adv.)

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Answer:

Correct Answer: 23. (d)

Solution:

Tangent to P1 passes through (2f2,0) i. e. (4,0).

T1:y=m1x+2m10=4m1+2m1m12=1/2

Also, tangent to P2 passes through (f1,0) i.e. (2,0).

T2:y=m2x+(4)m20=2m24m2m22=21m12+m22=2+2=4



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