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Question: Answered & Verified by Expert
If $b$ and $c$ are the lengths of the segments of any focal chord of a parabola $y^2=4 a x$, then the length of the semi-latus rectum is :
MathematicsParabolaJEE Main
Options:
  • A $\frac{b c}{b+c}$
  • B $\sqrt{b c}$
  • C $\frac{b+c}{2}$
  • D $\frac{2 b c}{b+c}$
Solution:
2567 Upvotes Verified Answer
The correct answer is: $\frac{2 b c}{b+c}$
Since the semi latus rectum of a parabola is the harmonic mean between the segments of any focal chord of the parabola. $\therefore l$ is the harmonic mean between $b$ and $c$. Hence,
$l=\frac{2 b c}{b+c}$

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