C superscript 2 subscript 5 = 5 choose 2.
=5×4÷2
= 10
A (up 2 down 5)=5*4.
A (upper m and lower n) is equal to n times the number of consecutive m.
C (upper 2 and lower 5)=5*4/( 1*2).
C (upper m and lower n) is equal to the product of consecutive m numbers starting from n divided by the product of consecutive m numbers starting from 1.
Definitions and formulas
Definition of arrangement: from N different elements, arbitrarily choose m(m≤n, m and n are natural numbers, the same below) to arrange in a column in a certain order, which is called an arrangement of taking M elements from N different elements; All permutation numbers of m(m≤n) elements from n different elements are called permutation numbers of m elements from n different elements.