TITLE: Parabolic subalgebra
QUESTION [6 upvotes]: Let $R$ a root system and $\Delta$ be a simple system of roots of a Lie algebra $\mathfrak g$, $\Delta'\subset \Delta$ and $R(\Delta')=R\cap \mathbb Z(\Delta')$. Define 
$$p(\Delta')=\mathfrak h \bigoplus_{\alpha \in R(\Delta')} \mathfrak g_{\alpha}  \bigoplus_{\alpha \in R^+ \setminus R^+(\Delta')}\mathfrak g_{\alpha}$$ the parabolic subalgebra associated to $\Delta'$.
If $\alpha$ is a simple root in $R^+(\Delta)\setminus R^+ (\Delta')$, then $\beta(h_\alpha)=0$ for all $\beta$ in $R(\Delta')$???

REPLY [5 votes]: The answer is NO.
Take $g=sl(3)$ and $\Delta=\{a_1,a_2\}$. The unique choice for $\Delta'$ is $\{a_2\}$.
The result is clearly false in this scenery.