Is every non-vertical straight line a function?

Answer 1

Yes - every non-vertical straight line passes the vertical line test, as a non-vertical straight line intersects with a vertical straight line at one or less locations.

The strict definition of a function is f: x->f(x) meaning that each value of f maps each value of x to exactly one value f(x).

For example, the square root is NOT a function, because the square root of any positive number yields two real answers (or two complex numbers for negative square roots).

It should be noted that in very advanced math, there are things called multivalued functions which are similar to normal functions, but are NOT the same.

It should also be noted that the vertical line test only applies to Cartesian Equations, not Parametric Equations or Polar Equations which you may learn about in your high school Advanced Algebra/Precalculus class or College Algebra class.