# Question #47d0a

If $A$ and $B$ are non null hermitian then $\text{tr} \left({A}^{2}\right) > 0$ and $\text{tr} \left({B}^{2}\right) > 0$ so $\text{tr"(A^2) +"tr} \left({B}^{2}\right) > 0$
so if ${A}^{2} + {B}^{2} = 0 \to A = 0 , B = 0$