Let $R$ be a reduced, irreducible, crystallographic root system with positive roots $R^+$ and simple roots $\Delta$. Let $W$ be the Weyl group of $R$. Let $(-,-)$ be a $W$-invariant scalar product on $\mathbb{R}\Delta$ (this is unique up to non-zero scalar as far as I know).

**Question 1**

Let $\alpha,\beta\in R^+$ such that $(\alpha,\beta)\geq 0$ and such that $\alpha+\beta\in R$. Is then always $(\alpha+\beta)^\vee=\alpha^\vee+\beta^\vee$?

Here we denote as usual by $\alpha^\vee=2\alpha/(\alpha,\alpha)$ the dual root / coroot of a root $\alpha$.

**Question 2**

Let $\alpha,\beta\in R^+$ such that $(\alpha,\beta)=0$ and such that $\alpha+\beta\in R$. Is then always $(\alpha+\beta)^\vee=\alpha^\vee+\beta^\vee$?

The reason why I cannot answer this question myself is that I am not familiar with pairs of roots which are orthogonal but are not strongly orthogonal. We say $\alpha,\beta\in R$ are strongly orthogonal if $\alpha\pm\beta\notin R\cup\{0\}$.

I understand that such pairs of roots can only occur if there are two root length and if $$ (\text{long root length})^2=2\cdot(\text{short root length})^2\,, $$ i.e. only in type $\mathsf{BCF}$. But still I am not able to find them. I cannot find roots for which the hypotheses are satisfied.

NB. It also counts as an answer if you give some examples of such pairs of roots and verify **Question 2** on them. In any case, please use the numbering of the simple roots as in the Bourbaki tables. It makes it much easier to follow for me.

Thank you very much for any kind of help!

Question 1andQuestion 2by the inequality $(\alpha+\beta)^\vee\leq\alpha^\vee+\beta^\vee$ where $\leq$ is the usual partial order on $\mathbb{Z}\Delta$ resp. $\mathbb{Z}\Delta^\vee$. $\endgroup$ – user66288 Sep 9 '16 at 17:36