$\displaystyle\Delta{A}{B}{C}\usepackage{cmbright}\Huge\$ ~ $\displaystyle\Delta{D}{E}{F}\usepackage{cmbright}\Huge\$ because $\displaystyle\frac{{4}}{{6}}=\frac{{5}}{{7.5}}=\frac{{6}}{{9}}\usepackage{cmbright}\Huge\$ .