High school mathematics and permutation and combination

A is not at both ends, only at positions 2 and 3.

Case 1: When A is in position 2, B can only be in position 1. There are two arrangements of cd: 2× 1 = 2.

Case 2: When A is in position 3, B can be in position 1 and position 2. After ab permutation, there are only two permutations of cd, so there are 2× 2 = 4.

To sum up, * * * has 4+2=6 combinations.

Papers on the history of mathematics

Forced to break up is just an illusion. Lu Yu unveiled the past in "Ning and Xiu". Have we always misunderstood flax?

How many volumes does Henan college entrance examination take in mathematics?

Self-taught undergraduate mathematics

Is there a Montessori primary school in Dalian?

How to test high school mathematics 125? At ordinary times 150 can get full marks around 1 10.

What are the teaching methods and strategies of primary school mathematics?

The incomparable author of the Uss circle

Second grade knows the time in Mandarin.

Textbooks and books for studying advanced mathematics, discrete mathematics, data structures and algorithms.