Question

If f(x) = 3x^2+ 3x^(1/2) + 3x, then f(-9) is equal to

Answer 1

Since

$f(x)=3x^2+3\sqrt{x}+3x$

the domain of f(x) is the set of non-negative integers, because we can't feed a square root with negative inputs. So, you couldn't comput f(-9)

Unless you're allowed to use complex numbers, in that case we have

$f(-9)=3\cdot (-9)^2+3\sqrt{-9}+3\cdot (-9)=3\cdot 81+3\cdot (9i)-27$

Which evaluates to

$243+27i-27=216+9i$