Let [;(A, \mu) ;] be a measure space (A is a sigma-algebra). If [; A_k ;] is a countable collection of sets in [; A ;], prove that [; \mu(\cup_{k=1}^{k=\infty}A_k) \leq \sum_{k=1}^{k=\infty} \mu(A_k) ;].

Can someone break this down into steps? As in, first prove this, then this, then....