Is to take out two at a time, (count one and take out two; Count to two and take out two more; Count to three and take out two more; ......)
It is called "two numbers in two places" and two elements become a combination.
Explanation 2: Two numbers in two places represent such numbers: 2, 4, 6, 8, 10.
Suppose there are six 1, and our two 2 numbers are the first two 1, the middle two 1 and the last two 1. The count is 2, 4, 6. It only takes three times to count six 1, but it takes six times to count one by one.
Shǔshù, with both verbs and noun parts of speech, refers to the calculation of detailed numbers in some way or way, usually with a wrench finger or mouth to recite spells or meditation, which is a relatively common and ordinary mathematical behavior.
Extended data:
Taking out M different elements (0≤m≤n) from N different elements at a time and synthesizing a group regardless of their order is called selecting the combination of M elements from N elements without repetition. The total number of all such combinations is called the combination number, and the calculation formula of this combination number is
or
The combination obtained by repeatedly extracting m elements from N-ary set A is essentially an M-ary subset of A ... If set A is ordered,
Become an ordered set, then the combination of m elements extracted from A corresponds to several segments.
A definite strict order-preserving mapping to ordered set A, the number of combinations? What are the common symbols?
Baidu encyclopedia-combination