Find the inverse of the matrix $\displaystyle I_n+ab^T$.

I know that I'm really suppose to work a little bit on my own first before asking questions here... But I don't really know how to start this problem here other than just writing up the matrices in this manner - the (i,j) entry being $\displaystyle 1+a_i^T*b_j$ where $\displaystyle a_i^T$ is the ith row of a and $\displaystyle b_j$ is the jth column of b.

Then the next question is finding the inverse of the matrix $\displaystyle D+ab^T $ where D is a diagonal matrix.

I suppose the first question will help me with this second?

Thank you!!!