Foto: "<a href="https://www.ccby.de/foto/teaching-writing-and-explaining-the-discriminant-formula-in-quadratic-equations/1063100" target="_blank">Teaching writing and explaining the discriminant Formula in quadratic equations</a>" von <a href="https://www.ccby.de/fotograf/marco-verch" target="_blank">Marco Verch</a> via <a href="https://www.ccby.de" target="_blank">ccby.de</a>, <a href="https://creativecommons.org/licenses/by/2.0/de/" target="_blank">CC-BY 2.0</a>