Combinatorics in grade 9
Combinatorics studies how to count configurations without listing everything. The sum rule (m+n, disjoint options) and product rule (m⋅n, independent stages) appear often.
Then: formulas for permutations Pn=n!, arrangements Ank (order matters), and combinations Cnk (order ignored).
First pick the model: “or” → sum; “and in stages” → product; then permutation, arrangement, or combination
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