4. Let n ≥ 8 be an even integer and let k be an integer with...

4. Let n ≥ 8 be an even integer and let k be an integer with 2 ≤ k ≤ n/2. Consider k-element subsets of the set S = {1, 2, . . . , n}. How many such subsets contain at least two even numbers?

Expert Solution

Since $n\geq 8$ is an even integer, let n=2m. And let k be an integer such that $2\leq k\leq m$ . Now the set S has 2m elements, so from S we can choose k elements in $^{2m}C_k$ ways, i.e., S has $^{2m}C_k$ numbers of k-element subsets.

Now to find the number of subsets containing atleast two even integers, we find the number of subsets which contains no even integer or exactly one even integer. Now observe that S has exactly m odd integers and exactly n even integers. So to find the number of subsets containing no even integer (that is, the subset contains odd integers only), we have to choose k elements from m odd integers of S, which can be chosen in $^{m}C_k$ ways. Hence, the number of subsets of S that contains no even integer is $^{m}C_k$ . Now let's find the number of subsets containing exactly one even integer. So the subset will contain exactly one even integer ( which can be any of m even integers of S) and k-1 odd integers (which has to chosen from m odd integers of S). So the even integer can be chosen in m ways and the k-1 odd integers can be chosen in $^{m}C_{k-1}$ ways. Hence, the number of subsets containing exactly one even integer is $m\cdot {^{m}C_{k-1}}$ .

So the number of subsets of S that contains less than 2 even integers is $^mC_k+m\cdot {^{m}C_{k-1}}$ and hence the number of subsets that contains atleast 2 even integers is $^{2m}C_k-\left (^mC_k+m\cdot {^{m}C_{k-1}}\right )$ where $m=\frac{n}{2}$ .

Colby Messinger answered 2 years ago

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