Обратные функции

InverseFunction[Cos]

InverseFunction[Log[Tan[#]]&][y]

InverseFunction[(2#-3)/(#-1)&][y]

InverseFunction[Sin[#^2]&][y]

InverseFunction[ConditionalExpression[Sin[#^2],0< #< Sqrt[Pi/2]]&][y]

Reduce[Sin[x^2]==y,x,Reals]