typo; fixed error in deriviate

cheat-sheets/analysis/Analysis_Wichtige_Formeln.tex

@@ -1,7 +1,7 @@
 \documentclass[a4paper,10pt]{article}
 \usepackage{amssymb}
 \usepackage{amsmath}
-\DeclareMahOperator\arctanh{arctanh}
+\DeclareMathOperator{\arctanh}{arctanh}
 \usepackage[utf8]{inputenc} % this is needed for umlauts
 \usepackage[ngerman]{babel} % this is needed for umlauts
 \usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf
@@ -71,7 +71,7 @@ e^x &= \sum_{n = 0}^{\infty} \frac {x^n}{n!} \\
 	(\arctan x)' &= \frac {1}{1 + x^2} \\
 	(\sin x)'    &= \cos x \\
 	(\cos x)'    &= -\sin x \\
-	(\arctanh x)' &= \frac {1}{\sqrt {1 + x^2}}
+	(\arctanh x)' &= \frac {1}{1 + x^2}
 \end{align*}