Voici une solution. \documentclass[10pt]{article} \usepackage[french]{babel} \documentclass[10pt,french]{article} \usepackage{lmodern} \usepackage[T1]{fontenc} \usepackage{babel} \usepackage[a4paper,left=20mm,right=20mm,top=15mm,bottom=15mm]{geometry} \usepackage{amssymb} \usepackage{amsmath} \DecimalMathComma % pour les virgules dans les nombres \begin{document} \[ \left\{ \begin{array}{lll} \alpha_{1}-\alpha_{2}-\alpha_{3}=0\\ \alpha_{1}-\alpha_{3}=0\\ \alpha_{1}+\alpha_{2}=0\\ \end{array} \right.\implies \left\{ \begin{array}{ll} \alpha_{1}-\alpha_{2}-\alpha_{3}=0\\ \alpha_{1}=-\alpha_{2}\\ \alpha_{1}=\alpha_{3}\\ \end{array} \right. \implies \left\{ \begin{array}{ll} \alpha_{1}=0\\ \alpha_{2}=0\\ \alpha_{3}=0\\ \end{array} \right. \] \end{document}

Voici une solution. \documentclass[10pt]{article} \usepackage[french]{babel} \usepackage{lmodern} \usepackage[T1]{fontenc} \usepackage[a4paper,left=20mm,right=20mm,top=15mm,bottom=15mm]{geometry} \usepackage{amssymb} \usepackage{amsmath} \DecimalMathComma % pour les virgules dans les nombres \begin{document} \[ \left\{ \begin{array}{lll} \alpha_{1}-\alpha_{2}-\alpha_{3}=0\\ \alpha_{1}-\alpha_{3}=0\\ \alpha_{1}+\alpha_{2}=0\\ \end{array} \right.\implies \left\{ \begin{array}{ll} \alpha_{1}-\alpha_{2}-\alpha_{3}=0\\ \alpha_{1}=-\alpha_{2}\\ \alpha_{1}=\alpha_{3}\\ \end{array} \right. \implies \left\{ \begin{array}{ll} \alpha_{1}=0\\ \alpha_{2}=0\\ \alpha_{3}=0\\ \end{array} \right. \] \end{document}