1: x/a + y/b = 1


This is one of the most basic equation of a straight line and you would do well to understand the simple properties. 

The x and y intercepts are (a,0) & (0,b)
The other results come from applying trigonometry to the various right angled triangles in the diagram. 
A perpendicular is dropped from the Origin to the line and its length is named as p. The angle this perpendicular makes with the x axis is named as alpha.
Thus you have these right angled triangles with p and alpha. And our results follow. Do them yourself and finally put into your memory as a graphical memory, structural memory.