Hyperbola
Focus and Directrix
JEE Main 2019
Grade 11

Question:

If $5x + 9 = 0$ is the directrix of the hyperbola $16x^2 - 9y^2 = 144$, then the corresponding focus is:
(1) $(-5, 0)$
(2) $(5, 0)$
(3) $(-\frac{5}{3}, 0)$
(4) $(\frac{5}{3}, 0)$

Step-by-Step Solution

Key Concept: Hyperbola: $x^2/9 - y^2/16 = 1; a^2 = 9, b^2 = 16, c = 5$. Directrix $x = -a^2/c = -9/5$ (nearer to left focus $(-5, 0)$). The given directrix $5x + 9 = 0 \Rightarrow x = -9/5$, so corresponding focus is $(-5, 0)$.
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Correct Answer: (1)

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