I'm reading Susanna Epp's book on discrete mathematics. In the section on logical equivalences she gives 11 laws, of which the first five are: $$Commutative: p\land q\equiv q\land p $$$$Associative: (p\land q)\land r\equiv p\land (q\land r)$$ $$Distributive: p\land (q\lor r)\equiv (p\land q)\lor(p\land r)$$$$Identity: p\land\top\equiv p$$$$Negation: p\lor \lnot p\equiv \top$$ I have only written out for $\land$, but of course there are the same laws for $\lor$. These are described as axiomatic and used to derive the other six laws. My question is how do you derive the law below from the five above? $$Absorption: p\lor (p\land q)\equiv p$$
2026-04-12 15:12:59.1776006779
Derive $p\lor (p\land q)\equiv p$ from the usual boolean algebra axioms
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I feel a bit silly answering my own question, but I guess it's for anyone that is interested.
$$p\lor (p\land q)$$$$\equiv (p\land \top)\lor (p\land q)$$$$\equiv (p\land (q\lor \lnot q))\lor (p\land q)$$$$\equiv ((p\land \lnot q)\lor(p\land q))\lor (p\land q)$$$$\equiv (p\land \lnot q)\lor ((p\land q)\lor(p\land q))$$$$\equiv (p\land \lnot q)\lor (p\land q)$$$$\equiv p\land (\lnot q \lor q)$$$$\equiv p\land \top$$$$\equiv p$$
Through some insightful comments it was pointed out that I used this law without justifying it with the five axioms: $$p\lor p \equiv p$$
Here is the solution I came up with:
$$p\lor p$$$$\equiv (p\lor p)\land \top$$$$\equiv (p\lor p)\land (p\lor \lnot p)$$$$\equiv p\lor (p\land \lnot p)$$$$\equiv p\lor \bot $$$$\equiv p$$