So why don't they prove for other quadrants? Is there's some neat trick, an argument for not doing that? Down below there's an image from an old textbooks (I tried modern ones, they are much worse) with a proof of equation for a line. But why? I mean a point in the I quadrant has different properties that a point in the II, right? So in the first, x>0 and y>0 but in the second x<0. So they prove for a point in one of the quadrants and then say "So for any point on the line we have that.". So what I mean is this: we have a line that passes through any 3 quadrants but textbooks prove the statements only for one of those quadrants. Here's where my problem lies: Why do authors only prove the statement for a point that lies in the I quadrant? (or only one point in any other quadrant). What a typical textbook would do is to draw a line, then take a point on a line in the I quadrant and then do some constructions and finally prove the statement.
So here's my problem: Imagine we want to proof some statement about a line, for example it's equation.
0 kommentar(er)
