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Combined Equation of a Pair Lines

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Estimated time: 13 minutes
Maharashtra State Board: Class 12

Key Points: Combined Equation of a Pair Lines

General Equation Combined equation of a pair of lines through the origin Combined equation of a pair of lines not passing through the origin
  ax² + 2hxy + by² = 0 ax² + 2hxy + by² + 2gx + 2fy + c = 0
Necessary Conditions for Real Lines h² − ab ≥ 0 \[\begin{vmatrix} \mathrm{a} & \mathrm{h} & \mathrm{g} \\ \mathrm{h} & \mathrm{b} & \mathrm{f} \\ \mathrm{g} & \mathrm{f} & \mathrm{c} \end{vmatrix}=0,\]
h² − ab ≥ 0
Point of intersection (0, 0) \[\left(\frac{\mathrm{hf-bg}}{\mathrm{ab-h^2}},\frac{\mathrm{gh-af}}{\mathrm{ab-h^2}}\right)\]
Angle between the lines \[\tan\theta=\left|\frac{2\sqrt{\mathrm{h}^2-\mathrm{ab}}}{\mathrm{a}+\mathrm{b}}\right|\] \[\tan\theta=\left|\frac{2\sqrt{\mathrm{h}^{2}-\mathrm{ab}}}{\mathrm{a}+\mathrm{b}}\right|\]
For parallel (coincident) lines h² − ab = 0 h² − ab = 0,
bg² = af²,
\[\frac{\mathrm{a}}{\mathrm{h}}=\frac{\mathrm{h}}{\mathrm{b}}=\frac{\mathrm{g}}{\mathrm{f}}\]
For perpendicular lines a + b = 0 a + b = 0
Maharashtra State Board: Class 12

Formula: Sum and Product of Slopes

\[m_1+m_2=-\frac{2h}{b}\]

\[m_1m_2=\frac{a}{b}\]

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