como calculo arc tan (9.9)

Escriba atan(9.9) 

= $$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}^{\!\!\mathtt{-1}}{\left({\mathtt{9.9}}\right)} = {\mathtt{84.232\: \!111\: \!102\: \!086^{\circ}}}$$