Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Combination reaction is a reaction in which two or more substances combine together to form a single substance.
$$A+B \rightarrow AB $$

Decomposition reaction is a reaction in which a compound splits into two or more simpler substances (elements or compounds).
$$AB \rightarrow C+D+..... $$
This is why Decomposition Reaction is called opposite of Combination Reaction.
$$ CaCO_{3} (S) \rightarrow CaO (s) + CO_{2}(g) $$
This is a Decomposition reaction $$ H_{2}(g) + Cl_{2} \rightarrow 2HCl (g) $$
This is a Combination reaction