quality:correction, stylistic:tone
Subtraction principle

Similarly, for a given finite set S, and given another set A, if A ⊂ S {\displaystyle A\subset S}, then | A c | = | S | − | A | {\displaystyle |A^{c}|=|S|-|A|}. To prove this, notice that | A c | + | A | = | S | {\displaystyle |A^{c}|+|A|=|S|} by the addition principle.