Given that a and B are real numbers, the size of a ^ 2 + B ^ 2 and ab is Do you know the situation

Given that a and B are real numbers, the size of a ^ 2 + B ^ 2 and ab is Do you know the situation

Of course, the situation!
1: When a, b > = 0
a*a+b*b>=2ab
2: When a * b2ab
3: When a, B = 2Ab
In conclusion:
a*a+b*b>=2ab