 Solution

In a semi-additive category, hom-sets are commutative monoids and composition is additive in both variables. Write $S={\coprod }_i{A}_i$, $P={\prod }_i{A}_i$, with injections ${\nu }_i$ and projections ${\pi }_i$. The canonical $c:S\to P$ has ${\pi }_{i}c{\nu }_j=1_{A_i}$ when $i=j$ and zero otherwise. DefineBilinearity givesThe universal properties of the product in a category and coproduct in a category imply $cd=1_P$ and $dc=1_S$. For the empty family, $S$ is initial and $P$ terminal. Their endomorphisms are necessarily the identity and the zero morphism; the zero map $P\to S$ therefore also gives inverse composites. Thus the comparison is an isomorphism for every finite family, giving a biproduct: