I'm reading a book named "Geometry, A comprehensive course" by Dan Pedoe at the moment. But I don't understand when they talk about the equation of a straight line...
Here is what is written:
The lines of our plane are sets of points which satisfy equations of the form $$a_1X_1+a_2X_2+a_3=0$$
Could someone explain me what the variables of the equations stand for and what it actually is?
Thanks in advance