Question: Find the angle between the lines represented by the following equation: x² + 5xy + 6y² = 0.

Solution:
Given,

Single equation of two straight lines is 
$x²+5xy+6y²=0$
$or, x² + 2×\frac{5}{2} xy + 6y²=0$ - (i)

Comparing equation (i) with ax² + 2hxy + by²= 0, we get,
$h = \frac{5}{2}, a = 1, b = 6$

Angle represented by the following single equation is given by:

$tan \theta = \left ( \pm \dfrac{2 \sqrt{h^2 - ab}}{a +b} \right )$

$or, tan \theta = \left ( \pm \dfrac{2 \sqrt{ (\frac{5}{2})^2 - 1×6}}{1 +6} \right )$

$or, tan \theta = \left ( \pm \dfrac{ 2\sqrt{\frac{25}{4} - 6}}{7} \right )$

$or, tan \theta = \left ( \pm \dfrac{2 \sqrt{ \frac{25 -24}{4}}}{7} \right )$

$or, tan \theta = \left ( \pm \dfrac{2 \sqrt{ \frac{1}{4}}}{7} \right )$

$or, tan \theta = \left ( \pm \dfrac{2 × \frac{1}{2}}{7} \right )$

$or, tan \theta = \left ( \pm \dfrac{1}{7} \right )$

Taking positive sign,
$or, tan \theta = \frac{1}{7}$
$or, \theta = tan^{-} \frac{1}{7}$
$\therefore \theta = 8°$ [Use calculator]

Taking negative sign,
$or, tan \theta = - \frac{1}{7}$
$or, \theta = tan^{-1} ( - \frac{1}{7} )$
$\therefore \theta = 172°$ [Use calculator]

Therefore, the required angle between two lines represented by the given single equation of two lines is 8° or 172°.