Prove that if $0\lt a\lt b$, then $a^2\lt \frac{1}{3}\left(a^2+ab+b^2\right)\lt b^2$.

Lesson Parent: 
Problem: 

Prove that if $0\lt a\lt b$, then $a^2\lt \frac{1}{3}\left(a^2+ab+b^2\right)\lt b^2$.

Answer: 

It is true that if $0\lt a\lt b$, then $a^2\lt \frac{1}{3}\left(a^2+ab+b^2\right)\lt b^2$.