linear algebra - Evaluate determinant of an $n times n$-Matrix

I have the following task:
Let $K$ be a field, $n \in \mathbb{N}$ and $a,b \in K^n$.
Evaluate the determinant of the following matrix:

$$\begin{pmatrix}
a_1+b_1 & b_2 & b_3 & \dots & b_n& \\
b_1 & a_2 + b_2 & b_3 & \dots & b_n \\
b_1 & b_2 & a_3 + b_3 & \dots & b_n \\
\vdots & \vdots & \vdots& & \vdots \\
b_1 & b_2 & b_3 &\dots & a_n + b_n
\end{pmatrix}$$

What I did was expanding it as follows using the Laplace expansion:

$$\det A =(a_1 + b_1) \det\begin{pmatrix} a_2 + b_2 &\dots& b_n \\ b_2 &\dots& b_n \\ \vdots & & \vdots \\ b_2 & \dots & a_n + b_n \end{pmatrix} - b_2 \det\begin{pmatrix} b_1 & b_3 &\dots& b_n \\ b_1 & a_3 + b_3 &\dots& b_n \\ \vdots & \vdots & & \vdots \\ b_1 & b_3 & \dots & a_n + b_n \end{pmatrix} + \ b_3 \det\begin{pmatrix} \dots \end{pmatrix} - \dots (-1)^{n+1}b_n \det\begin{pmatrix} b_1 &\dots& b_{n-1} \\ b_1 &\dots& b_{n-1} \\ \vdots & & \vdots \\ b_1 & \dots & b_{n-1} \end{pmatrix}$$

And before I expand the rest of those determinants and fill 20 papers with a's and b's I'd like to ask for advice. Is this the right way? And when I think about it I don't really see any simplification that is possible when I have finally expanded everything to a point where I could use Cramers rule.
It just came to my mind that I could also expand using the Lapace rule by iterating through the rows instead of the columns. By doing that I'd be able to factor out all of those $b_1$...

NOTE: I am not allowed to use the Sylverster Determinant Theorem

Thank you very much for your help.

FunkyPeanut

Answer

$$\begin{array}{ll}
D_n&=\begin{vmatrix}
a_n+b_n & b_{n-1} & b_{n-2} & \dots & b_1& \\
b_n & a_{n-1} + b_{n-1} & b_{n-2} & \dots & b_1 \\
b_n & b_{n-1} & a_{n-2} + b_{n-2} & \dots & b_1 \\
\vdots & \vdots & \vdots& & \vdots \\
b_n & b_{n-1} & b_{n-2} &\dots & a_1 + b_1
\end{vmatrix}\\

&=\begin{vmatrix}
a_n & b_{n-1} & b_{n-2} & \dots & b_1& \\
0 & a_{n-1} + b_{n-1} & b_{n-2} & \dots & b_1 \\
0 & b_{n-1} & a_{n-2} + b_{n-2} & \dots & b_1 \\
\vdots & \vdots & \vdots& & \vdots \\
0 & b_{n-1} & b_{n-2} &\dots & a_1 + b_1
\end{vmatrix}\\
&+\begin{vmatrix}
b_n & b_{n-1} & b_{n-2} & \dots & b_1& \\
b_n & a_{n-1} + b_{n-1} & b_{n-2} & \dots & b_1 \\

b_n & b_{n-1} & a_{n-2} + b_{n-2} & \dots & b_1 \\
\vdots & \vdots & \vdots& & \vdots \\
b_n & b_{n-1} & b_{n-2} &\dots & a_1 + b_1
\end{vmatrix}\\
&=a_nD_{n-1}+b_n\begin{vmatrix}
1 & b_{n-1} & b_{n-2} & \dots & b_1& \\
1 & a_{n-1} + b_{n-1} & b_{n-2} & \dots & b_1 \\
1 & b_{n-1} & a_{n-2} + b_{n-2} & \dots & b_1 \\
\vdots & \vdots & \vdots& & \vdots \\
1 & b_{n-1} & b_{n-2} &\dots & a_1 + b_1

\end{vmatrix}\\
&=a_nD_{n-1}+b_n\begin{vmatrix}
1 & 0 & 0 & \dots & 0 & \\
1 & a_{n-1} & 0 & \dots & 0 \\
1 & 0 & a_{n-2} & \dots & 0 \\
\vdots & \vdots & \vdots& & \vdots \\
1 & 0 & 0 &\dots & a_1
\end{vmatrix}\\
&= a_n D_{n-1}+b_n\prod_{k=1}^{n-1}a_k
\end{array}$$