a)
Deoarece a < b si a,b apartine lui R => a²<ab.
La fel si in cazul al doilea: a < b => ab < b².
=> a² < ab < b².
b)
a < b => a + b < 2b => (a+b)/2 < b.
a < b => 2a < a + b => a < (a + b)/2.
Concluzie,(a + b)/2 apartine (a,b).
