A math problem from the first semester of junior high school 25

Let this project be 1, i.e., the speed of pair A is 1 10, and the speed of team B is 1 20, according to the title.

Solution: 3 * (1/10 + 1/20) = 91/20 - 9/20 = 11/20

1/10 * (1 + 1/3) = 2/15

2/15 + 1/20 = 11/60

11/20 divided by 11/60 = 3

A: It will take 3 days.

Solve with engineering problem ideas:

A originally worked at a working efficiency of 1 10

B works at a rate of 1 20

After 3 days of cooperation, the two have done 3 * (1 10 + 1 20) = 9 20, and have not done 1-9 20 = 11 20

After the improvement, the work efficiency of A is 1 10 * (1 + 1 3) = 2 15, then the number of days is (11 20) (2 15 + 1 20) = 3

If this project is regarded as unit 1, then team A will do 1 10 a day, and team B will do 1 20 a dayBecause the two teams do it for three days, multiply by 3They are then subtracted from unit 1 to get 11/20 of the remaining works

Next, the efficiency of the question is increased by 1/3, so the efficiency of Team A is 1/30 (multiply 1/10 by 1/3).Then use the current efficiency of team A of 1/30 and the efficiency of team B of 2/20 to get the efficiency of the two of them in a day. Finally, divide 11/20 by the sum of efficiency, and the result is equal to days.

11 20 (1 10 1 3 1 20) days).

1》Let type A be the Y unit, then the B type is the 100-Y unit. The equation for simultaneous inequality is as follows:

47500 =< (2800-2200) y + 3000-2600)(100-y)<=480000

Solving the inequality yields < y<=40

That is, there are three schemes: 1) 38 units for type A and 62 units for type B;

2) 39 units of type A and 61 units of type B;

3) 40 sets of type A and 60 sets of type B;

2) Because the cost of B is high and the profit is low. So option 3 is the best.

**The subsidy is: (2800 * 40 + 3000 * 60) * 13% = 37960 yuan = 10,000 yuan.

Set up A-type X-stage and B-type Y-stage.

47500"=600x+400y"=48000 x+y=100xy are all integers greater than or equal to 0.

Solution: 2x》=75 y》=60 x=38 39 40 y=62 61 60

Three scenarios. x=40 y=60 The cost is minimal.

40 * 2800 + 60 * 3000) * 13 100 = 37960 subsidy 37960 yuan.

(1) Set up the production of type A X units, then type B is 100-X units, and the profit YY = 600X + 400 (100-X).

When 47500 < y <48000, < x < 40

Therefore, there are two schemes: a38, b62 or a39, b61: (2): 2200*38+2600*62, < 39*2200+61*2600

Therefore, the cost of plan B is lower, and the cost is 244400 **, which requires a subsidy of 31772 yuan.

The general formula is:

2)^(n-1)*x^n*y

The 9th item will be 9 into the Zheng clan group to lift oranges to get 256x 9y

The coefficient and number of times are like spikes.

The coefficient is (-2) (n-1).

The number of times is n+1

1) 258x nine-game defeat to the power of the Ming withered y

2) (-2) n-1 x to the nth power

The coefficient and the number of times you tell you should know as long as you are not a white.

The answer to the first question is 592x to the ninth power y The answer to the second question is a mononomial of -2 (n-1) and the good bridge god is n to the n power of y, and the number of times is n!! 1

The solution of the equation 2x+1/3a=x-1 satisfies 2x+3=7.

This means that the value of x in 2x+3=7 is the same as the value of x in the previous equation.

From 2x+3=7 we get x=2 , and we get the previous equation :

4 + 1/3a = 1

Solution A = 9

Based on 2x+3=7, x=2 is obtained

Bring x=2 into 2x+1/3a=x-1

Get a=7

Solution: Let's say the growth rate of oil ** this month is x relative to the previous month. According to the question (1 x) (1 5%) = 1 14%.

Solution: x=20% Answer: This month's oil** has a growth rate of 20% compared to the previous month

Comment: This question is a growth rate application problem. The cost of importing oil this month is equal to the previous month's cost plus the increased fee, which is the amount of oil imported this month multiplied by the current month's **.

Set the unknowns, represent each quantity, and list the equations to solve. The key to solving the problem of column equations is to find the relationship between the equal quantities, and then use the algebraic formula to express the quantities in it, and the column equation is solved.

If the previous month** is A, and the import volume of the previous month is B, then the cost of importing oil in the previous month is ab

If the import volume of this month is (1-5%)b, and the cost of importing oil this month is (1+14%)ab, then the ** of imported oil this month is 114%ab 95%b = Then, the growth rate of this month's oil ** relative to the previous month is.

Equation (8x+40) 3-30=9x 5

x=250/13

2) The apprentice brushes 450 13 per day, the master brushes 840 13 per day, one master and two apprentices brush 1740 13 per day, and the area of 36 rooms is 9000 13, which takes 6 days.

My understanding is this:

Excluding the loss in the machining process, it can be seen that this problem needs to be solved according to the volume of the ingot.

Solution: From the meaning of the question, it can be seen that before and after processing, the volume of the steel ingot remains unchanged, so the inner diameter of the steel pipe after processing is r, there is.

6 2) 1= 3 2) -r ] i.e. 9=(

The simplified shift item is:

The solution is r= cm

Therefore, the answer is D

To do it with the principle of constant volume, first of all, the volume of the ingot is 900 900 divided by 450 centimeters long, which means that the area is equal to 2

So the square of the outer radius diameter, minus the square of the inner radius is 2 so the square of the inner radius is centimeters, and the answer is d

BV cylindrical = r 2 h = 3 3 100

Thickness of steel pipe = v cylinder (450 3).

Inner diameter = 3 - steel pipe thickness 2

Comrades. The inner diameter is the diameter, not the radius, multiply the answer by 2, not an integer, and the small answer should be close to the first, cccccc

It doesn't have to be one more lap, it should be set to run n laps.

Then 450x=250x+400n, where n is an integer.

Then x=400n 200=2n

Then the first encounter is n=1, that is, 2 minutes later, and the second encounter is 4 minutes later, so to help you solve it is to tell you that some questions require several encounters.

It is known that Xiaohu ran 450x, Xiaohu ran 250x, and Xiaohu ran more than Xiaobei for one lap, which is 400 meters, so the equation can be obtained.

450x-250x=400

200x=400

The problem of x=2 is not very difficult, as long as you draw a diagram, it is clear.

450x-400=250x

He runs one lap more than him, so subtracting the distance of one lap gives you a few minutes of encounter.

1 Full solution: If there are x students and the complete fare is unit 1, then according to the proposal, there is an equation as follows:

So it turns out that there are students x = 21 people.

Then now after joining A student, A travel agency fee:

B Travel Agency Fee:

This is where the discussion is needed.

Let's assume that a travel agency is cheaper.

That is, there is an inequality <

Solve a> but the number of people can't be decimalized, only rounded!

That is to say, when A is more than one person, it is more appropriate to choose A travel agency, and when A = 1, it is more appropriate to choose B:

As for why this is so, it is according to this inequality relation! Although it was hypothetical, then we discussed the scope of such a hypothesis!

Heuristics are the application of inequalities and practical problems, important considerations, experimentation and practicality!

Good luck! You don't have a bounty point, I'm still typing for a long time, hope vortex! Have fun!