In the book Number Theory IV from Parshin, one can find this statement (precisely at p. 215) with the comment "it is easy to see that...":
Let $a_{ij}$ ($1\le i,j\le m$) be complex numbers with $|a_{i,j}|\le H_i$ ($1\le j\le m$). One assumes that the linear formforms $L_i(\underline X)=\sum_{i=1}^ma_ix_i$ are linearly independent. Then, for any $\overline w=(w_1,\cdots,w_m)\in\mathbb C^m$, one has $$\sum_{i=1}^m\frac{L_i(\overline w)}{H_i}\ge c_1\frac{\Delta}{H_1\cdots H_m}$$$$\sum_{i=1}^m\frac{|L_i(\overline w)|}{H_i}\ge c_1\frac{|\Delta|}{H_1\cdots H_m}$$ where $\Delta=\det(a_{i,j})$ and $c_1=\frac1{m!}\max_{1\le j\le m}|w_j|$.
Unfortunately, for me it is not easy. Can one have hints to prove that? Thanks in advance