Consider the eigenvector $\mathbf{u_i}$ corresponding to the eigenvalue $\lambda_i = 0$ for the matrix $AA^{\dagger}$, for an arbitrary $M \times N$ matrix $A$. Hence:
$$AA^{\dagger}\mathbf{u_i} = 0$$
Show that if $\mathbf{b}$ is in the range of A (i.e. $A\mathbf{x}=\mathbf{b}$)
then:
$$\mathbf{u_i}^{\dagger} \mathbf{b} = 0$$
My attempt:
\begin{align}AA^\dagger \mathbf{u_i} &= 0 \\ \mathbf{u_i}^\dagger AA^\dagger &= 0 \tag{1} \\ \mathbf{u_i}^{\dagger} \mathbf{b} &= \mathbf{u_i}^\dagger A\mathbf{x} \tag{2}\end{align}
I am unsure how to link $(1)$ and $(2)$ because it is not given that A is unitary, so i cannot assume $A^{\dagger}A = I$
Any help is appreciated!