A and B trains leave AB at the same time, and meet 75 km away from a for the first time. After meeting, they continue to move forward and return to their destination immediately, The second meeting is 55 meters away from B. how far is ab?

It can be close or close,
When the two cars met for the second time, they made a total of three journeys. The first one was 75 kilometers, and the third one was 75 kilometers
75 × 3 = 225 (km)
The second meeting was 55 meters away from B, which means that a traveled 55 kilometers after B. the distance of AB is as follows:
225-55 = 170 (km)

Classification of rational numbers in the first year of junior high school
1. Positive integer set package does not include 0?
2. Does nonnegative integer set package not include negative fraction set?
3. Natural numbers include 0?
Is the cycle of - 1.3 a negative fraction?

1. Excluding 0
0 is neither positive nor negative, so it can't be counted in a positive rational number
2. Not included
Non negative integers are positive integers, not including negative fractions
3. Natural numbers include 0
4. Yes
An infinite circular decimal is a fraction

The distance between city a and city B is 560 km. The two trains leave from city a and city B at the same time and meet in 3.5 hours. Car a travels 85 km per hour, and how many km per hour does car B travel?

560 △ 3.5-85, = 160-85, = 75 (km); answer: car B travels 75 km per hour

Given that the average of a group of data - 2, - 1, 0, x, 1 is 0, then the variance of this group of data is 0______ ．

∵ the average of data - 2, - 1, 0, x, 1 is 0, ∵ (- 2-1 + 0 + X + 1) △ 5 = 0, the solution is x = 2, ∵ the variance of this group of data is: S2 = 15 [(- 2-0) 2 + (- 1-0) 2 + (0-0) 2 + (2-0) 2 + (1-0) 2] = 2; so the answer is: 2

A car goes from place a to place B at the speed of 100 km / h. after arriving at place B, it drives back from place B to place a at the speed of 60 km / h. The average speed of this car is______ Kilometers

(1 + 1) △ 1100 + 160, = 2 △ 8300, = 75 (km); a: the average speed of this car is 75 km

How many squares are there from 1 to 1999

The smallest, of course, is 1
44²

A and B start from a and B at the same time, and run in opposite directions. After 4 hours, they meet. After meeting, a continues to drive for 3 hours to B. B travels 60 kilometers per hour. How many kilometers are there between a and B?

A's speed: 60 × 4 △ 3 = 80 (km / h), total distance: (60 + 80) × 4 = 560 (km). A: the distance between a and B is 560 km

As shown in the figure, in the quadrilateral ABCD, AB "CD, e and F are the midpoint of AC and BD respectively

Let's talk about it briefly
Let g in AD connect Ge and GF
Then Ge is parallel to CD, GF is parallel to AB, Ge = CD / 2, GF = AB / 2
When CD is parallel to AB, GE and gf are on the same line, EF = gf-ge = (ab-cd) / 2
When CD is not parallel to AB, G, e and f form a triangle,
Then EF > | gf-ge | = | AB / 2-CD / 2 | = | ab-cd | / 2, because AB > CD, EF > (ab-cd) / 2
In conclusion, EF > = (ab-cd) / 2

For a pile of coal, first use 1 / 5 of the total amount, then use 4 / 9 of the total amount, and there is 80 left. How much coal is there

1 / 5 + 4 / 9 = 29 / 45
29 out of 45 = 16 out of 45
80 divided by 16 / 45 = 225 (how much coal is there)

Mean value inequality solution function
f（x）=（x²-x=1）/（x-1）（x＞1）
F (x) = (X & # 178; - x + 1) / (x-1) (x > 1) wrong number

f(x)=(x²-x)/(x-1)+1/(x-1)
=x+1/(x-1)
=(x-1)+1/(x-1)+1
x>1,x-1>0
So f (x) ≥ 2 √ [(x-1) * 1 / (x-1)] + 1 = 2 + 1 = 3
The minimum is 3

A and B trains leave AB at the same time, and meet 75 km away from a for the first time. After meeting, they continue to move forward and return to their destination immediately, The second meeting is 55 meters away from B. how far is ab?