How do you find the y-intercept of a function?

Hello!
The y-intercept of a function is the point of intersection of the graph of this function and the y-axis. So such a point belongs both to y-axis and to the graph of a function.
All points on the y-axis have an x-coordinate of 0, they are of the form (0, y), where y may be any number.
All points on the graph of a function f(x) have the form (x, f(x)), where x is any number from the domain of f.
If both conditions are met, we have x = 0 and the second coordinate is f(0), hence the y-intercept of f is the point (0, f(0)), it is unique if function is one-valued. Of course this requires that 0 is in the domain of f, otherwise f has no y-intercept.
For example, f(x)= x^2+1 has the y-intercept (0, 1), and g(x) = 1/x has no y-intercept.
More general curves, not graphs of (one-valued) functions, may have more than one y-intercept, for example the circle x^2+y^2=1 has two (find them yourself using x=0 ).