(a) Combination reactions occur when two or more substances combine to producece a single substance or product, or when two reactants combine to form a single product.
(b) Burning of carbon in presence of oxygen to produce carbon dioxide is an example of a combination reaction which is also an exothermic reaction.
C (s) + O2 (g) →CO2 + Heat
(c) Reaction of Nitrogen with oxygen to give nitrous oxide is an example of a combination reaction which is also an endothermic reaction.
N2 (g) + O2 (g) + Heat → 2NO (g)