Math problems see the supplement for details, and reward for correct answers .

Suppose you buy all 28 parts, spend 28 * 600 = 16800 yuan, mp4 buy 0 parts, spend 0 yuan, buy *** than buy mp4 costs 16800 yuan;

Then for every less ***, buy more MP4, buy *** than buy MP4 less than spend 600 + 900 = 1500 yuan, so the actual number of buy less ***, that is, the actual number of MP4 is: (16800-1800) 1500 = 10, buy *** is: 28-10 = 18.

Look at this satisfaction not.

If you buy back *** x, then MP4 is 28-x

Solution: 600x-900 (28-x)=1800 Divide both sides by 300 at the same time.

2x-3×(28-x)=6

2x+3x-84=6

5x=90x=18 28-x=28-18=10 so the school buys back *** 18 units, mp4 10 units.

Set: ***x, MP4 is 28-x

600x-900（28-x）=1800

X, i.e. 18 units for purchase *** and 10 units for MP4.

Let *** be xmp4 and y, and let the binary equation be 600x-900y=1800x+y=28, and solve x=28-y600(28-y)-900y=1800600*28-600y-900y=180016800-1500y=1800-1500y=1800-16800-1500y=-15000-y=-10y=10y=28-10x=18

1. The amount of grass growing per day is x

If there is a y in the grassland, then (y+24*x) 70*24=(y+60*x) 30*60 simplification: y=480x

Therefore, the daily growth of grass is one-480th of the original (the result cannot be calculated)4 Set A x, B y, 2x + 2y = 42; 14x=42+14y x=12 y=9

5. Teams A, B and C complete all the projects separately, and day C 1 a+1 b=1 6,1 b+1 c=1 10,1 a+1 c=1 5

Additive: 1 a+1 b+1 c=7 30

Subtraction: c = 15

b=30a=15/2

6. Each team of A, B and C needs 15, 30, 15 2 days to complete all the projects separately, and set up x square meters for the table and y square meters for the table legs.

x+y=5x*50*4=y*300

Solution: x=5-y

5-y)*200=300y

1000=500y

y=2x=3

3 square meters of table top, 2 square meters of legs.

7 seems to be wrong, only said **, not at all.

83.67 million square kilometers).

I'm tired, count it yourself, give it a point.

(1) Let x: the amount of grass eaten per cow per day Set y: the amount of grass grown per day 24 70x=1+24y: 30 60x=1+60y The joint solution yields y=slightly.

4) Let A x, B y, 2x+2y=42; 14x=42+14y ok

5) Oh, I have limited time, the questions are not difficult, they are all elementary school questions, set x, y, and joint conditions!

I'm leaving, sorry!

Hit rate = number of hits divided by number of shots multiplied by 100 100 Xiao Li's hit rate = 46 divided by 50 times 100 100 = 92% Xiao Ming's hit rate = 28 divided by 30 times 100 100 = so: Xiao Ming's high.

46 50-28 30=-2 150<0, so Li Ming's hit rate is high.

The solution of the equation for the first problem.

Question 2:

A value is taken arbitrarily two, a = 1, and the equation is 2x -2 = 0

a=2, the equation is 3x -y - 1 = 0 to get x = 1, y = 2

Bringing x = 1, y = 2 to (a+1)x-(a-1)y+a-3=0, we get a+1-2(a-1)+a-3 =0 regardless of the value of a.

x = 1, y = 2 is the common solution.

Question 3: At most n (n 1) 2 intersection points, any two straight lines intersect, the analysis process is as follows: if any one line L is taken among the n straight lines, then l and the remaining n 1 straight lines intersect, and there are at most n 1 intersection points on l.

In the same way, there is a maximum of n 1 intersection points on each line.

So n lines have a maximum of n*(n 1) intersections, but the intersection of any two straight lines is calculated again (one straight line once) so n straight lines have a maximum of intersections n*(n 1) 2 at least 0 intersections, and any two straight lines are parallel to each other.

The solution to the fourth question.

There is another solution to the third problem.

If n>=2.

When n=2, two straight lines intersect in pairs to get up to 1 intersection point, and when n=3, three straight lines intersect in pairs to get up to 1+2=3 intersection points, (the third straight line intersects with each of the previous lines).

There are 4 straight lines in the plane that intersect in pairs, and a maximum of 1+2+3=6 intersection points can be obtained, (the 4th straight line intersects with each of the previous lines).

There are 5 straight lines in the plane that intersect in pairs, and you can get up to 1+2+3+4=10 intersection points, (the 5th straight line intersects with each of the previous lines).

For each additional line, make it intersect with all the lines that precede it.

Therefore, the number of intersections is 1+2+3+4+5+...n-1=(1+n-1)[This is the algorithm of flashback, the first term plus the last term Number of terms 2](n-1)[Number of terms] 2

The final result is n(n-1) 2

1. 5 days for A, 4 days for B, 1 day for C.

Two x=1 y=2

3. No, it won't. 4. A 123 B 97 C 177

1. Use the hypothetical method to solve this problem.

Method 1: Assuming that all of them are ***, then buy *** more than mp4:

600 28 16800 (yuan), extra 16800 1800 15000 (yuan), mp4 has: 15000 (600+900) 10 (parts).

Yes: 28 10 18 (part).

Method 2: Assuming that all are mp4, then buy *** less than mp4:

900 28 25200 (yuan), a total of less 25200 + 1800 27000 (yuan), *** have: 27000 (600 + 900) 18 (part).

MP4 has: 28 18 10 (part).

Second, with the equation method (but the solution of the equation in this method is beyond the scope of the primary school) set mp4 to buy x part, then *** has (28-x) part.

600×(28-x)-900x=1800

The solution is x=10

28-10=18 (part).

Set *** to buy x parts, mp4 to buy y parts.

x+y=28

600x-900y=1800

The solution is x=18 y=10

I'll just write you the sum of the equations, and if you need the idea and the solution of the equation, then ask me the one you can't figure out.

The original salt of a bottle of brine is one-eleventh of water, after adding 15 grams of salt, the salt accounts for one-ninth of the brine, how many grams of the original brine in the bottle?

Solution: If the original salt water x, then there is 10x water

x+15)*9=(x+15)+10x

x = 60 (grams).

Original salt water = original water Original salt 60 * 10 + 60 = 660 (grams).

Two barrels of oil A and B, a barrel of oil is heavier than B barrel of oil, after pouring out grams from each of the two barrels of oil, five-tenths of the twenty-first of a barrel is equal to one-third of a barrel B, how much does the two barrels of oil of A and B weigh respectively?

Solution: If the original weight of the first barrel of oil is x grams, then the original weight of the second barrel of oil is grams.

x=18, so barrel A of oil originally weighed 18 grams, and barrel B of oil originally weighed grams.

Processing a batch of parts, originally planned to process 30 per day, when the processing of one-third, due to the improvement of technology, the work efficiency increased by 10 percent, the result was completed 4 days ahead of schedule, how many parts are there in this batch?

Solution: There are x parts.

x/30-4=(x*1/3)÷30+(x-x*1/3)÷(30+30*10%)

x=1980 (pcs).

If two cars A and B drive out of A and B at the same time, they will go in the opposite direction. You can meet you on the way after 4 hours. It is known that the speed ratio of car A and B is 3:

4. At this speed, if the two cars have to meet at the midpoint of A and B. How many hours in advance should car A depart?

Solution: The car should be set out x hours in advance.

4*(3+4)/2=3*(14/4+x)

x=1 and 1 6

That is, A should be opened 1 hour and 10 minutes in advance.

1,000 kilograms of fruit with a moisture content of 99 percent were transported to the warehouse, and a week later it was found that the water content had been reduced to 98 percent.

Solution: Let's say that this batch of fruits weighs x kilograms.

1000-1000*99%=x-x*98%

x = 500 (kg).

Two cars A and B travel in the opposite direction from the two places at the same time, and then the two cars meet at a distance of 20 meters from the midpoint, and it is known that the speed of A is three-quarters of that of B, and the speed of car B is obtained.

If B's velocity is x, then A's velocity is 3x 4

x=64(m-h).

Hello, 1After one salt addition, the water remains unchanged, and the condition in the question can be changed to the score of salt to water.

1 11-1 =1 10 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

So water + salt = 600g + 60g = 660g

1. ab=ac, bp=aq, abp= acq, it can be known: abp acq (a corner between the two sides).

2. From 1, we can know: AP=AQ, CAQ= BAP, we can know: BAC= PAQ=60°, we can get: APQ is an equilateral triangle.

3. It can be known: pq=cq=aq=ap=bp, acp bcp, acp= bcp=30°, cbp=180°-100°-30°=50°

abp=∠bap=60°-50°=10°，∠apb=180-2*10°=160°