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sagarsabnis
If x is a positive integer, f(x) is defined as the number of positive integers which are less than x and do not have a common factor with x other than 1. If x is prime, then f(x) =?

A. x-2
B. x-1
C. (x+1)/2
D. (x-1)/2
E. 2

We are given that f(x) is defined as the number of positive integers that are less than x and do not have a common factor with x other than 1. We also are given that x is a prime number and we must determine the value of f(x).

Since x can be any prime number, let’s let x = 5. The positive integers less than 5 are 1, 2, 3, and 4. Note that those integers do not have a common factor with 5, other than 1. Thus, f(x) = 4.

Plugging 5 in for x in our answer choices, we see that the only answer choice that equals 4 is B, x - 1.

Answer: B
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How x-1 true in case of prime number 2?

Between 2 & 1 there are zero positive numbers and 2-1 = 1
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How x-1 true in case of prime number 2?

Between 2 & 1 there are zero positive numbers and 2-1 = 1

Bunuel

If \(x\) is a positive integer, \(f(x)\) is defined as the number of positive integers smaller than \(x\) that share no common factors with \(x\) other than 1. If \(x\) is a prime number, what is the value of \(f(x)\) in terms of \(x\)?


A. \(x - 2\)
B. \(x - 1\)
C. \(\frac{(x + 1)}{2}\)
D. \(\frac{(x - 1)}{2}\)
E. 2


The question is essentially asking how many positive integers smaller than a given prime number \(x\) have no factors in common with \(x\), except for 1.

Since \(x\) is a prime number, it only has two distinct factors: 1 and itself. As a result, any positive integer smaller than \(x\) will not share any factors with \(x\) other than 1. Therefore, there will be \(x-1\) positive integers smaller than \(x\) that meet this criterion.

For example, consider the prime number \(x=7\). The positive integers smaller than 7 that have no factors in common with 7, except for 1, are: 1, 2, 3, 4, 5, and 6. Thus, the value of \(f(x)\) for \(x=7\) is \(7-1=6\).


Answer: B
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How x-1 true in case of prime number 2?

Between 2 & 1 there are zero positive numbers and 2-1 = 1

Bunuel

If \(x\) is a positive integer, \(f(x)\) is defined as the number of positive integers smaller than \(x\) that share no common factors with \(x\) other than 1. If \(x\) is a prime number, what is the value of \(f(x)\) in terms of \(x\)?


A. \(x - 2\)
B. \(x - 1\)
C. \(\frac{(x + 1)}{2}\)
D. \(\frac{(x - 1)}{2}\)
E. 2


The question is essentially asking how many positive integers smaller than a given prime number \(x\) have no factors in common with \(x\), except for 1.

Since \(x\) is a prime number, it only has two distinct factors: 1 and itself. As a result, any positive integer smaller than \(x\) will not share any factors with \(x\) other than 1. Therefore, there will be \(x-1\) positive integers smaller than \(x\) that meet this criterion.

For example, consider the prime number \(x=7\). The positive integers smaller than 7 that have no factors in common with 7, except for 1, are: 1, 2, 3, 4, 5, and 6. Thus, the value of \(f(x)\) for \(x=7\) is \(7-1=6\).


Answer: B

If x = 2, the number of positive integers smaller than 2 that share no common factors with 2 other than 1 is one, and that integer is 1, which does not share any common factor with 2 except 1.
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If \(x\) is a positive integer, \(f(x)\) is defined as the number of positive integers smaller than \(x\) that share no common factors with \(x\) other than 1. If \(x\) is a prime number, what is the value of \(f(x)\) in terms of \(x\)?


A. \(x - 2\)
B. \(x - 1\)
C. \(\frac{(x + 1)}{2}\)
D. \(\frac{(x - 1)}{2}\)
E. 2

M23-35

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why is number 1 counted since it is explicitly stated that it dosen't share any other factor except 1, it should be excluded.
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sagarsabnis
If \(x\) is a positive integer, \(f(x)\) is defined as the number of positive integers smaller than \(x\) that share no common factors with \(x\) other than 1. If \(x\) is a prime number, what is the value of \(f(x)\) in terms of \(x\)?


A. \(x - 2\)
B. \(x - 1\)
C. \(\frac{(x + 1)}{2}\)
D. \(\frac{(x - 1)}{2}\)
E. 2

M23-35

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why is number 1 counted since it is explicitly stated that it dosen't share any other factor except 1, it should be excluded.

You're misunderstanding the phrase "other than 1" in the question. It doesn’t mean to exclude 1 from the count. It simply clarifies that the only common factor these numbers have with x is 1. If x is 7, then the number of positive integers smaller than x that share no common factors with x other than 1 is six: 1, 2, 3, 4, 5, and 6.
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