Question

why is f(-1/2) undefined?

Answer 1

what is the function or graph? That question doesn't make sense without more information.

Answer 2

A function is often undefined at some value if that value would cause you to have to divide by zero.

So, like if your function was f(x) = 1/(x-3), then f(3) would be undefined because making x = 3 causes a zero in the denominator.