The associative property states that $\displaystyle{\left({a}+{b}\right)}+{c}={a}+{\left({b}+{c}\right)}\usepackage{cmbright}\Huge\$ . Therefore, $\displaystyle{\left({4}+{5}\right)}+{1}={4}+{\left({5}+{1}\right)}\usepackage{cmbright}\Huge\$ , and the equation has been rewritten using this property.