Answer:
4 is always true
Step-by-step explanation:
I'm not sure if there's a faster way to do this, but you can randomly substitute numbers in and eliminate choices.
One and two can be immediately eliminated, because x can be any number and there are no squares, exponents, etc. If there were a x² and +9 it will always be a positive number (negative number squared will always be positive), but in this case there isn't.
Since there is a +9, we know that if x is a positive number f(x) will always be postitive. For statement 3 however, if you substitute a negative number that is too "small" for example -3, you get -1+9 which is 8. This doesn't fufill the requirement of "If x < 0 then f(x) < 0".
