The first time they met at a distance of 7 km from B After meeting, they continue to drive forward. When they arrive at their destination, they return immediately. When they return, they meet at a distance of 4km from A. how many kilometers is the distance between a and B?

The first time they met at a distance of 7 km from B After meeting, they continue to drive forward. When they arrive at their destination, they return immediately. When they return, they meet at a distance of 4km from A. how many kilometers is the distance between a and B?

The distance between the two places is 17 kilometers
The reason is that when they meet for the first time, they make a whole journey together. When they meet for the second time, they make three whole journeys. We also know that when they make a whole journey together, B makes 7 kilometers. When they make three whole journeys together, B should make 21 kilometers
The distance between the two places is 17 km
(7*3+4)/2
The first time we met, B traveled 7 kilometers
Second encounter, three whole journey, then B line 7 × 3 = 21 km
AB distance = 21-4 = 17 km
17 km
A and B start from a and B at the same time and run in opposite directions. They meet at 5km away from the midpoint. The speed ratio of a and B is 7:8. Find the distance between a and B
10 * 15 = 150 km
The two vehicles of a and B are going from AB to ab at the same time. The meeting point of the two vehicles is at the midpoint of 8km. It is known that the speed of a is 1.2 times that of B. the distance between a and B is calculated
I'll know when I'm here. Can you hurry up? That's true
8 × 2 = 16 km
Distance of line B: 16 ÷ (1.2-1) = 80 km
Then the whole journey: 80 × (1 + 1.2) = 176km
If the speed of B is x, then the speed of a is 1.2x, then there is
(Y / 2 + 8) △ 1.2x = (Y / 2-8) △ x, the solution is y = 176km
Finding a sequence of four numbers
There are four numbers. The first three items are equal difference series, the last three items are equal ratio series. The sum of the first and the fourth items is 16, and the sum of the middle two items is 12
Let the four numbers be: A-D, a, a + D, (a + D) ^ 2 / A
Then A-D + (a + D) ^ 2 / a = 16
a+a+d=12
The solution is a = 4, d = 4 or a = 9, d = - 6
So the four numbers are: 0,4,8,16 or 15,9,3,1
Zero
Four
Eight
Sixteen
On a straight road, there are two places a and B, which are 150 kilometers apart. A and B patrol cars start from a and B at the same time, and drive to B and a at a constant speed. The speeds of a and B patrol cars are 70 km and 80 km / h respectively, and the driving time is set to be x hours, How many kilometers is the distance between the two cars? It is expressed by the algebraic formula containing X. ② it is known that both cars are equipped with walkie talkies, and each walkie talkie can talk to each other within 15 kilometers (including 15 kilometers). How long can the two walkie talkies keep talking during driving?
1. A travels 70X km from a and B 80x km from B, so the distance is [150 - (70X + 80x)] km, that is (150-150x) km (0
Four large trucks and six small trucks carry a total of 35 tons of goods. It is known that the carrying capacity of large trucks is twice that of small trucks. How many tons of goods are transported by each truck
6+（24）=14
35/14=2.5
2.52=5
Big truck 5 small truck 2.5
What are the definitions of sequence, sequence and set, and what are the differences between them?
The sequence is arranged according to a certain grade
A sequence of numbers arranged in a certain order is called a sequence of numbers
A set of numbers
The distance between a and B is 1200 km. Two cars leave at the same time. Car a travels 90 km per hour and car B 80 km per hour. After a few hours, the distance between a and B is 180 km?
(1) (1200-180) / (90 + 80) = 1020 / 170 = 6 (hours) (2) (1200 + 180) / (90 + 80) = 1380 / 170 = 13817 (hours) answer: after 6 hours or 13817 hours, the distance between a and B vehicles is 180 km
The two trucks will weigh 580 tons more than a construction site
There were two more trucks, and each truck had to transport the soil five times before it was finished. There were several large trucks
There are n large trucks
10 ×n=580
10 × n divided by 10 = 580 divided by 10
n=58
58-2 = 56 vehicles
That's 56 trucks
How to use the sequence construction method,
Sequence construction method can solve many difficult problems, but it is not absolutely easy to use
Example 1: A1 = 1, an + 1 = 2An + 3 * (1 / 2) ^ (n + 1)
It is very important that we have the same construction form
[an + 1 + p * (1 / 2) ^ (n + 1)]
We get P = 1
[an + (1 / 2) ^ (n)]
Example 2: known positive number sequence: Nan - (n + 1) a (n + 1) = 2n (n + 1) an * an + 1, find an, n ∈ n*
This problem, together with the above one, is made up by myself, which is more complicated
However, it is not difficult to find that n (n + 1) on both sides has repetition, so the division on both sides, n ∈ n *, can be divided. And the same thing is, an * a (n + 1) and n (n + 1) above are the same repetition, and are also positive numbers, division
When you do division, you are happy: 1 / (n + 1) * a (n + 1) - 1 / N * an = 2. The original 1 / N * an is an arithmetic sequence of reciprocal numbers
In this problem, a big formula is very frightening, slightly deformed, and consider in the direction of reciprocal, about repeating the symmetric term and formula
I'll give you two examples first. If you have any questions later, just ask me and my team