Q.If the line joining the two points $A(2, 0)$ and $B(3, 1)$ is rotated about $A$ in the anticlockwise direction through an angle of $15^\circ$, then find the equation of the line in the new position.

Angle Between Two Lines

In space, the angle between two lines is measured through their directions, not their positions — two lines that never meet still have a well-defined angle between them (the angle you would see if you slid one across to meet the other).

So the angle between the lines is just the angle between their direction vectors. If the lines run along $\vec{b_1}$ and $\vec{b_2}$,

Why the absolute value

A line has two opposite directions, so $\vec{b}$ and $-\vec{b}$ describe the same line. The modulus in the numerator picks the acute angle ($0^\circ \le \theta \le 90^\circ$), which is the convention for the angle between lines.

In Cartesian form

If the lines have direction ratios $(a_1,b_1,c_1)$ and $(a_2,b_2,c_2)$,

If instead you know the direction cosines $(l_1,m_1,n_1)$ and $(l_2,m_2,n_2)$, the denominators are both $1$ and $\cos\theta = |l_1l_2+m_1m_2+n_1n_2|$.

Two special cases

• Parallel: the direction ratios are proportional, $\dfrac{a_1}{a_2}=\dfrac{b_1}{b_2}=\dfrac{c_1}{c_2}$.
• Perpendicular: the dot product vanishes, $a_1a_2+b_1b_2+c_1c_2 = 0$.

Example …