As the title states, the question is: Prove that if a and b are positive real numbers, then $a + b \geq ab$
For this proof, I'm supposed to prove by contrapositive.
So, I get this as a general statement:
Assume $a + b < ab$, then we will prove that a and b are real numbers that are less than 0.
But I'm wondering where do I go from here? The only thing I can think of is if a is negative and b is negative, multiplying two negatives will be positive. But then how do I actually write a formal proof for this?