linear algebra - Constant term in the characteristic polynomial of A

a. Show that if $A$ is any $ n \times n$ matrix, the constant term in the characteristic polynomial of $A$ is $\operatorname{det}(A)$.

b. Deduce that $A$ is invertible if and only if $\lambda = 0$ is not an eigenvalue of $A$.

I know that the characteristic polynomial $P(λ) = \operatorname{det}( A - \lambda I_n )$ and $\lambda$ is an eigenvalue if and only if $P(λ) = \operatorname{det}( A - \lambda I_n )=0$ but I'm not sure where to go from there.

Answer

Hint:

From the definition we have:
$$
P(\lambda)=
\det \begin{bmatrix}
a_{11}-\lambda & a_{12} & \cdots &a_{1n}\\
a_{21} & a_{22}-\lambda & \cdots &a_{2n}\\
\cdots\\
a_{n1} & a_{n2} & \cdots &a_{nn}-\lambda\\

\end{bmatrix}=
(-1)^n(\lambda^n+c_1\lambda^{n-1}+c_2\lambda^{n-2}+\cdots c_n)
$$

take $\lambda=0$