Optimization problem

Question

I have a function $ f(x)= \vert x\vert ^3, x\in \Bbb{R^n} $ I have to compute the inverse of the Hessian by using the formula $(I+uu^T)^{-1} = I - {1\over 2}uu^T $, where $I$ is an identity $nxn$ matrix and and unit vector $u\in \Bbb{R^n} $. I have computed the Hessian but I don't understand how it's of the form $I + uu^T$. Thanks in advance.