Combinatorics: Factorials- Basic Definitions
If $$x$$ is a positive integer greater than 2, the product of $$x$$ consecutive positive integers must be divisible by which of the following?
>I. $$x- 1$$
>II. $$2x$$
>III. $$x!$$
At this point, you can choose answer choice E with a high degree of certainty that all of the statements must be true. If you have the time, take a look at the algebra and prove why the statements must be true:
The product of $$x$$ consecutive integers is divisible by $$x!$$, so statement III is definitely true. Now, $$x!$$ is
>$$1\times 2\times 3\times$$ … $$(x-1)\times x $$,
so $$(x-1)$$ and 2 are included in $$x!$$ (that is, provided that $$x$$ is greater than 2). Hence, this is the correct answer.
Incorrect.
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Yes, statements II and III are true. But what about statement I? Can
you find an example eliminating this statement?
Incorrect.
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Yes, statement I and III are true. But what about statement II? Can
you find an example eliminating this statement?
Incorrect.
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Yes, statement I is true. But what about statements II and III? Can you find an example eliminating these statements?
Incorrect.
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Yes, statement II is true. But what about statements I and III? Can
you find an example eliminating these statements?
Correct.
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__Plug In__ $$x = 3$$. In this case, the product could be $$3 \times 4 \times 5 = 60$$.
>I. 60 is divisible by $$x - 1 = 3 - 1 = 2$$.
>II. 60 is divisible by $$2x = 2 (3) = 6$$.
>III. 60 is divisible by $$x! = 3! = 3 \times 2 \times 1 = 6$$.
__Plug In__ another value of $$x$$ to be sure because the question says *must be*. This time try an even integer (such as $$x= 4$$). In this case, the product could be $$2 \times 3 \times 4 \times 5 = 120$$.
>I. 120 is divisible by $$x - 1 = 4 - 1 = 3$$.
>II. 120 is divisible by $$2x = 2 (4) = 8$$.
>III. 120 is divisible by $$x! = 4! = 4 \times 3 \times 2 \times 1 = 24$$.
I only
II only
II and III only
I and III only
I, II, and III
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