Grade seven Mathematics - 8-3 + 1-7, what do you read according to the meaning of "harmony"? What do you read according to the meaning of "operation"?

Grade seven Mathematics - 8-3 + 1-7, what do you read according to the meaning of "harmony"? What do you read according to the meaning of "operation"?

The sum of minus eight minus three plus one minus seven
Minus eight minus three plus one minus seven

-2 + 5-7, read according to the meaning of "he" and ()

-2 + 5-7, read according to the meaning of "he" (the sum of negative 2, 5 and 7), read according to the meaning of operation (negative 2 plus 5 minus 7)

The formula - 8 + 16-3-6 is read according to the meaning of "and"_____________ Read in the sense of operation_____________ .

Minus 8 plus 16 plus minus 3 plus minus 6. Minus 8 plus 16 minus 3 minus 6

Shanghai residence permit, social security base for three consecutive years more than 2 times, that is: 100.1 times more than 50 points

In recent three years, the average wage of employees in Shanghai is 3896 yuan in 2011, 4331 yuan in 2012 and 4692 yuan in 2013. When your social security payment base reaches the standard of that year, 50 points will be recorded and 100 points will be recorded if it reaches twice the standard of that year

What is the meaning of 100% miss rate in quantum statistics

The lower the miss rate of a page, the better. It means that the user will click the link to go to another page or close the window directly (that is, leave your store). The ratio is the number of people leaving your store / the total number of visits. Obviously, the value is 0 or very low, which means that the goods in your store

If a car travels at a speed of 15m / s for half of the whole journey, and then decelerates at a speed of 15m / s for the other half, then it just stops. The average speed of the whole journey should be: 10, but I can't work it out,
What's the average speed? What's the average speed?

The average velocity is the displacement divided by time,
Average speed is the distance divided by time, it is not the size of average speed (equal only when time approaches 0)
In this problem, the average speed of the whole course is v = s total / T total
Total T = T1 + T2
Constant velocity time T1 = (s total / 2) / V1 = s total / (2 * V1)
Uniform deceleration time T2 = (s total / 2) / average after v = (s total / 2) / (V1 / 2) = s total / v1
So t total = [s total / (2 * V1)] + [s total / V1] = 3 * s total / (2 * V1)
The average velocity of the whole course is v = (2 * V1) / 3 = (2 * 15) / 3 = 10 m / s

The average speed of the first half of the distance is 20m / s, and the average speed of the second half is 30m / s?

Let half the distance be X
The first half of the distance movement time: T1 = x / V1 = x / 20
The second half of the distance movement time: T2 = x / V2 = x / 30
The average velocity is equal to the displacement divided by the corresponding time
Average speed in the whole journey:
v=2x/(t1+t2)=24m/s

Someone has taken the following method to determine the average speed of a train: count from the first collision at the junction of wheel and rail, and the timing result is that 25 collisions are heard within 1 minute. Given that each rail is 25m long, what is the speed of the train during this period?

Every time the wheel arrives at the rail joint, it makes an impact sound. When the first sound is heard, it starts to count. A total of 25 impacts are heard. Therefore, the distance of the train is s = (25-1) × 25m = 600m, the running time of the train is t = 1min = 60s, and the speed of the train is v = st = 600m, 60s = 10m / s. A: the speed of the train is 10m / s

First question
When someone stands on the elevator, the time from the first floor to the second floor is T1. If the elevator doesn't move, the time from the first floor to the second floor is T2, then
The time t required for the elevator in operation from the first floor to the second floor is ()
2t1t2/t1+t2
I wonder why, v = s / T, how to do this? Why is the distance 2t1t2?
Question 2: an object passes 1 / 3 of a distance at speed V1, and then passes 2 / 3 of the distance at speed V2, so as to find the average speed of the distance
The answer is 3v1v2 / 2v1 + v2
This question seems to be similar to the previous one. First of all, I don't know why he wrote 3v1v2 like this. Why is it 2v1 + V2 instead of V1 + 2v2? Isn't it true that one is 1 / 3 and the other is 2 / 3?
Isn't the above two questions average speed? V = s / T, but he gives the variable speed and distance, how to calculate the average speed? And why is that distance expressed by multiplication
Finally, the average speed of the car from tunnel a to tunnel B is 40km / h, and from tunnel B to tunnel a is 50km / h
Why not 45? What's the right thing to do? "/
It's like asking too many questions,

First of all, we need to explain the concept of average speed. Average speed refers to the ratio of the distance passed and the time needed to pass this distance, which has nothing to do with the specific movement state. Therefore, we only need to know the distance and the time on this distance

When a car moves on a straight road, if the speed of the first half of the time is V1, and the speed of the second half is V2, what is its average speed in the whole journey? If the speed of the first half of the journey is V1 ', and the speed of the second half of the journey is V2', what is its average speed in the whole journey?

First, if half of the time is t, then the whole distance is (V1 + V2) t, and the average speed is v = (V1 + V2) t / 2T = (V1 + V2) / 2
Second, if half of the distance is s, then the total time is t = (s / V1 '+ S / V2') = s (V1 '+ V2') / v1'v2 ', and the average speed is v' = 2S / T = 2v1'v2 '/ (V1' + V2 ')
Anyway, you have to remember that the average speed is the total distance divided by the total time