1. The property used in expansion is the **distributive property** of multiplication over addition. 2. This property states that for any numbers $a$, $b$, and $c$, we have $$a(b+c) = ab + ac$$. 3. In expansion, each term inside the parentheses is multiplied separately by the term outside the parentheses. 4. For example, expanding $x(y+z)$ gives $$xy + xz$$ by distributing $x$ to both $y$ and $z$. 5. This property allows us to simplify expressions and solve equations by removing parentheses and combining like terms. 6. It is fundamental in algebra for expanding polynomials and simplifying expressions.