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10-2. Parabola, Ellipse, Hyperbola
hard
If ${m_1}$ and ${m_2}$are the slopes of the tangents to the hyperbola $\frac{{{x^2}}}{{25}} - \frac{{{y^2}}}{{16}} = 1$ which pass through the point $(6, 2)$, then
A
${m_1} + {m_2} = \frac{{24}}{{11}}$
B
${m_1}{m_2} = \frac{{20}}{{11}}$
C
${m_1} + {m_2} = \frac{{48}}{{11}}$
D
both $(a)$ and $(b)$
Solution
(d) The line through $(6,2)$ is
$y – 2 = m(x – 6)$
==> $y = mx + 2 – 6m$
Now from condition of tangency, ${(2 – 6m)^2} = 25{m^2} – 16$
==> $36{m^2} + 4 – 24m – 25{m^2} + 16 = 0$
==> $11{m^2} – 24m + 20 = 0$
Obviously its roots are ${m_1}$ and ${m_2}$, therefore
${m_1} + {m_2} = \frac{{24}}{{11}}$ and ${m_1}{m_2} = \frac{{20}}{{11}}$.
Standard 11
Mathematics