There are 5 students who need to shake hands when they meet. How many times do they shake hands? 100 people? M people?

There are 5 students who need to shake hands when they meet. How many times do they shake hands? 100 people? M people?

5 people 4 + 3 + 2 + 1 = 10 times
100 people 99 + 98 + 97 + +1 = (99 + 1) * 99 / 2 = 4950 times
M people 1 + 2 + 3 + +（m-1)=(1+m-1)(m-1)/2=m(m-1)/2

There are 20 students in the class. They should shake hands once and several times

190

There are 50 students in class one holding a party. If everyone shakes hands with other students once, how many times in total?

[(50-1) 50] △ 2 = 1225 (Times)
That is, 50 people and everyone except themselves shake hands once, and there is a repetition between each two people, so divide by 2
In the future, we can directly apply the formula [(n-1) n] △ 2
Hey, hey, O (∩)_ ∩)O