It all depends on what the objective of your graph is, although it is better to be safe then sorry...so if you can plot as many points as you can, it won't hurt.
No, the least amount of points you can do on a graph to see the pattern is 3 because than you can get a better line for your graph to see what kind of pattern your graph is.
An integer divided by its opposite always equals -1: this is false. Any number which is not 0 divided by it's opposite is in fact equal to -1, but since 0 is also an integer, it's false. You are right, good job!
