|a-b|=2
|a-b|^2=4
|a|^2-2a*b+|b|^2=4
|a|^2-2|a|*|b|*cos60+|b|^2=4
|a|^2-|a|*|b|+|b|^2=4
|a|*|b|=|a|^2+|b|^2-4>=2|a|*|b|-4
|a|*|b|<=4
a*b=|a|*|b|*cos60=1/2*|a|*|b|<=2
max=2