The most general form of a quadratic is: $ax^2 + by^2 + 2gx + 2fy + c + 2hxy= 0 $ and that for a homogeneous second degree equation is : $ax^2 + by^2 + 2hxy=0$
I derived the formula $$\tan \theta = \left|\dfrac{2\sqrt{h^2 - ab}}{a+b}\right|$$
but later, author claimed that the same formula is applicable for the general form too.
But I (and he too) derived it for the homogeneous case only. How can he claim this then?
On
The slopes of the lines are unaffected by the linear and constant terms of the equation. To see this, find the intersection point of the lines and translate the origin to this point. Doing so will eliminate the linear terms of the equation. This intersection can be found in various ways†, such as setting the partial derivatives to zero, yielding $$x_0={bg-fh\over h^2-ab}, y_0={af-gh\over h^2-ab}.$$ Setting $x=x'+x_0$ and $y=y'+y_0$ in the general equation produces $$ax'^2+by'^2+2hx'y'+{abc-af^2-bg^2+2fgh-ch^2\over h^2-ab}=0.$$ The numerator of the constant term is equal to the determinant of $$\begin{bmatrix}a&h&g\\h&b&f\\g&f&c\end{bmatrix}$$ which vanishes since this is a degenerate conic, leaving $$ax'^2+by'^2+2hx'y'=0.$$
† Another way is to perform Gaussian elimination on the above coefficient matrix. This will both give you the center point and also show that the constant term in the transformed equation must vanish for this null space to be non-trivial.
One can write $$0 = ax^2+by^2+2gx+2fy+c+2hxy = (a_1x+b_1y+c_1)(a_2x+b_2y+c_1)$$
So, the angle between two lines is $$\tan \theta = \left|\frac{\tan \alpha -\tan \beta}{1+\tan\alpha\tan\beta}\right| = \left|\frac{-\frac{a_1}{b_1}+\frac{a_2}{b_2}}{1+\frac{a_1a_2}{b_1b_2}}\right| = \left|\frac{-a_1b_2+b_1a_2}{a_1a_2+b_1b_2}\right|= \left|\frac{\sqrt{(a_1b_2+a_2b_1)^2-4a_1a_2b_1b_2}}{a+b}\right| = \left|\frac{\sqrt{4h^2-4ab}}{a+b}\right|.$$