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Question 1: Find the sum of consecutive natural numbers between 1 and 209?

Answer: The formula for summing with the equal difference series: s=[(1+209)*209] 2=21945 (the formula is [(1+n)*n] 2) n is the last natural number.

Question 2: There are 51 students in a class, and each of them shakes hands with other people once when they graduate, so how many times did they shake hands in total?

Use the combination method in the permutation and combination method: a total of 51 people, 2 people shake hands, it can be understood that from 51 people, take 2 people as a group, then use (c51 take 2) (high school knowledge) (c51 take 2 calculation method is 51 * 50 2 = 1275).

Question 3: Dad is 43 years old this year, and his son is 11 years old, and in a few years Dad will be 3 times older than his son?

Let's say: After n years, the father's age is 3 times that of the son, 43+x=3*(11+x) solves x=5 so it is 5 years later.

3. Solution: Set x years later.

43＋x=（11＋x）×3

43＋x=33+3x

10=2xx=5

Since I now use the equation with a multi-rake.

The third problem uses the 極 equation.

I forgot how to do it.

Question 1: 1+...100 = 100 2 * (100 + 1) = 5500 Question 2: 51 * 50 2 = 1275

Question 3: 43+x=3(11+x)x=5

1.The equation for the difference series is (1+209)*209 2=219452Pick one person to start with 50 times for the second person and 49 times for the first person (it should be the first person and he has shaken) and so on for the third person 48 times for the 4th person 47 So it's 50+49+48+47+++ to 1

The formula (50+1)*50 2=1275 so is 1275 times.

3 Let the father be 3 times older than the son after n years 43+x=3*(11+x) solves x=5 so it is 5 years later.

The total score of Chinese and English is 187 points, the total score of Chinese and Mathematics is 195 points, and the total score of Mathematics and English is 190 points, and the above 3 conditions are added together: 2 Chinese + 2 English + 2 Mathematics = 572 Then, 1 Chinese + 1 English + 1 Mathematics = 286

Language = 286-190 = 96

English = 286-195 = 91

Mathematics = 286-187 = 99

187 + 195 + 190 = 572 points is twice the sum of Chinese, Mathematics and English.

Therefore, the sum of Chinese, Mathematics and English is 286.

then the language score = 286-190 = 96 points.

Mathematics score = 286-187 = 99 points.

English score = 286-195 = 91 points.

Yang Qing gave Wang Zhaotian's three classmates 1 centimeter each, the total is 3 centimeters, plus 125 centimeters after being divided, it is 128 centimeters.

The total height of Yang Qing and the others (125x4) minus the total height (124x3) of Wang, Zhao, and Tian is Yang Qing's height.

i.e. 125x4-124x3 = 128 cm.

Assuming that Yang Qing's height is a, then the average height of the four people is 125=(124*3+a) 4, so that a one-dimensional equation is formed, and a = 128cm is calculated.

B moves the decimal point one place to the right, which is equal to multiplying by 10, i.e. A = B 10 sum of two numbers =, A + B = , and A = B 10

B 10 + B =

B = A = B 10 = 14

Billion becomes billion.

Billion becomes billion. It is known that A is equal to 10 times that of B.

A + B = 10 B + B = 11 B =

So B = A = 14