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Combinatorics

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Subsets of Cardinality $k$

COMBIN-ULD9VL

Which two expressions are equal to the number of $k$-element subsets of an $n$-element set? Select ALL that apply.

A

${n-1\choose k}+{n-2\choose k}$

B

${n-1\choose k-1}+{n-2\choose k-2}$

C

${n-1\choose k}+{n-1\choose k+1}$

D

${n-1\choose k-1}+{n-1\choose k}$

E

${n\choose k-1}+ {k\choose 1}$

F

${n\choose k}$