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Overview of Pair of Straight Lines

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Estimated time: 12 minutes
Maharashtra State Board: Class 12

Definition: Combined Equation

An equation representing two lines together is called the combined (joint) equation of the lines.

Maharashtra State Board: Class 12

Definition: Homogeneous Equation

An equation in which the degree of every term is the same is called a homogeneous equation.

Homogeneous equation of degree 2:

\[ax^2+2hxy+by^2=0\]

Maharashtra State Board: Class 12

Definition: Degree of a Term

The sum of the indices of all variables in a term is called the degree of the term.

Maharashtra State Board: Class 12

Key Points: Nature of Lines

Condition Nature
\[h^2-ab>0\] Distinct lines
\[h^2-ab=0\] Coincident lines
\[h^2-ab<0\] Not a pair of lines
Maharashtra State Board: Class 12

Formula: Slopes of the Lines

If \[ax^2+2hxy+by^2=0\]
Then slopes are:

\[m_1=\frac{-h-\sqrt{h^2-ab}}{b}\]

\[m_2=\frac{-h+\sqrt{h^2-ab}}{b}\]

Their sum is m1 + m2 = \[-\frac{2h}{b}\]

product is m1 m2 = \[\frac{a}{b}\]

Maharashtra State Board: Class 12

Definition: Auxiliary Equation

For \[ax^2+2hxy+by^2=0\]

Slopes of lines are roots of: \[bm^2+2hm+a=0\]

This equation is called the Auxiliary Equation.

Maharashtra State Board: Class 12

Formula: Angle Between Lines

\[\tan\theta=\frac{2\sqrt{h^2-ab}}{a+b}\]

Maharashtra State Board: Class 12

Key Points: Conditions for Perpendicular and Parallel

Sr. No. Condition Type Mathematical Condition Additional Result
1 Perpendicular Lines a + b = 0 Lines are perpendicular
2 Parallel Lines  \[h^2-ab=0\] Lines are parallel
3 Intersecting Lines \[h^2-ab\geq0\] Point of intersection is
\[\left(\frac{hf-bg}{ab-h^2},\frac{gh-af}{ab-h^2}\right)\]
Maharashtra State Board: Class 12

Definition: General Second Degree Equation

Equation of the form \[ax^2+2hxy+by^2+2gx+2fy+c=0\], where at least one of a,b,h is not zero, is called a general second degree equation in x and y.

The expression\[abc+2fgh-af^{2}-bg^{2}-ch^{2}\] is the expansion of the determinant \[\begin{vmatrix}
a & h & g \\
h & b & f \\
g & f & c
\end{vmatrix}\]

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