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Answers for “什么是公因数?”

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Yes, in this context, "greatest common factor" is being used for what is more commonly called the greatest common divisor in many modern texts. The mathematical procedure shown is the same subtraction-based Euclidean algorithm.

Conditions: Used informally in the explanation of why the algorithm works.

Meet the concept

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A common factor is a positive integer that divides each of the compared positive integers exactly. It appears in the positive-factor list of every number being compared.

Conditions: Compare two or more positive integers.; The common factor is positive and divides each target exactly.

Understand why

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The presenter factors out the common coefficient 23\frac{2}{3} to make the subsequent arithmetic simplification easier. After evaluating the antiderivative at the upper and lower bounds, both terms inside the brackets contain the factor 23\frac{2}{3}.

Conditions: The expression to evaluate is 49[23⋅93/2−23⋅13/2]\frac{4}{9}\left[\frac{2}{3}\cdot 9^{3/2}-\frac{2}{3}\cdot 1^{3/2}\right].; Both bracketed terms contain the same factor 23\frac{2}{3}.

Understand why

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A common divisor also divides the difference because division is interpreted as repeated subtraction. If a number divides evenly into the larger number and the smaller number, subtracting the smaller number repeatedly from the larger one will eventually leave a difference that the same divisor also divides evenly.

Conditions: There are two numbers in the example.; A chosen divisor divides evenly in the sense described by the speaker.; The subtraction is performed from the larger number using the smaller number.

Find a method

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To find the GCF by listing factors, list all positive factors of the first number, list all positive factors of the second number, identify the factors that appear in both lists, and select the largest number from the common factors.

Conditions: The inputs are positive integers.; Listing is practical for the small examples; it is not asserted to be the fastest method.

Find a method

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To start the Euclidean algorithm for gcd⁡(10,45)\gcd(10,45), you write the larger number as the smaller number multiplied by an unknown quotient plus an unknown remainder. Specifically, you set up the division equation 45=10⋅q+r45 = 10 \cdot q + r.

Conditions: The inputs are positive integers.; The larger number is placed on the left-hand side of the equation.; The quotient is an integer and the remainder satisfies 0≤r<100 \le r < 10.

Find a method

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To find the greatest common divisor of two large numbers, repeatedly apply the division-with-remainder step. Start by dividing the larger number by the smaller number.

Conditions: The inputs are two positive integers.; The division algorithm is applied at each step.; The process stops when a remainder equals 0.

Meet the concept

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The speaker verbally says "greatest common denominator," but the mathematical notation on the board is "gcd," which conventionally stands for "greatest common divisor." The context of dividing integers to find a common factor confirms that the intended concept is the greatest common divisor, and the spoken word is a verbal slip.

Conditions: The video discusses finding the common factor of two integers.; The board displays the notation gcd(a;b).; The procedure involves repeated integer division.

Understand why

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In the Euclidean algorithm, when the division process yields a remainder of zero, the greatest common divisor of the original two integers is the last non-zero remainder obtained. The algorithm stops at this point because the method is over, and the last non-zero remainder is guaranteed to divide both original numbers evenly.

Conditions: The Euclidean algorithm is applied to two integers.; The process of repeated long division is followed until a remainder of zero is reached.

Understand why

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The greatest common factor is useful for simplifying fractions. Dividing both the numerator and the denominator by their GCF reduces the fraction to its simplest form in one step, where the numerator and denominator have no common positive divisor greater than 1.

Conditions: The fraction has a positive-integer numerator and denominator.; The denominator is nonzero.

Understand why

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The example ends with 4 because applying the subtraction rule repeatedly yields 4 as the final value. First, 12−8=412 - 8 = 4, creating the pair 8 and 4.

Conditions: Start with the pair 12 and 8.; Repeatedly subtract the smaller from the larger.

Find a method

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Knowing the greatest common factor (GCF) of 12 and 42, which is 6, allows you to divide both the numerator and the denominator by 6 directly. This immediately yields the simplest form 2/72/7, bypassing intermediate steps like dividing by 2 first.

Conditions: The fraction is 12/4212/42.; The GCF of 12 and 42 is known to be 6.