One pool has a storage capacity of 20 cubic meters, and a water pipe is used to inject water into the pool at the speed of 5 cubic meters per hour. At the same time, another drainage pipe is used to drain water out of the pool at the speed of 6 cubic meters per hour. (1) write the relationship between the storage capacity V (M3) of the pool and the inflow time t (hour) (pay attention to the value range of the independent variable); (2) when is the water in the pool drained?

(1) V = 20 + 5t-6t = 20-t (0 ≤ t ≤ 20); (2) get 0 = 20-t from the meaning of the question, and get t = 20

When x approaches 0, KX ^ 2 is the equivalent infinitesimal of (√ 1 + xarctanx) - 1, then how does the constant K come from

15:13 × 13:3,6:5 × 12:7,4:3 × 9:2,19:1 × 49:76

(13/15)x(3/13)=5.
(5/6)x(7/12)=35/72.
(3/4)x(2/9)=1/6.
(1/19)x(76/49)=4/49.

4. In order to implement the central government's national policy of "energy conservation and emission reduction, beautify the environment, and build a beautiful new countryside", a village in our city plans to build 12 biogas digesters of type A and type B, so as to solve the fuel problem of all farmers in the village. The floor area, number of farmers and production price of the two types of biogas digesters are shown in the following table:
Model area
Number of households (unit: m2 / unit)
(unit: Household / unit) cost
(unit: 10000 yuan / piece)
A 15 18 2
B 20 30 3
It is known that the area of biogas digester is less than 220m2, and there are 288 households in this village
(1) How many schemes meet the conditions?
(2) By calculation, we can judge which construction plan is the most economical

(1) If X type a biogas digesters are built, then (20-x) type B biogas digesters will be built 1 point
According to the meaning of the title 3 points
The solution is 7 ≤ x ≤ 9 4 points
∵ x is an integer ∵ x = 7,8,9, ∵ there are three schemes to satisfy the conditions ∵ 5 points
(2) When X type a biogas digesters are built, the total cost is RMB 10000
y = 2x + 3( 20－x) = －x+ 60 ……………………………………………… 6 points
∵ - 1 < 0, y decreases with the increase of X,
When x = 9, the value of Y is the smallest, and y = 510000 yuan 7 points
At this time, the plan is to build 9 type a biogas digesters and 11 type B biogas digesters 8 points
Solution 2: from (1), we know that there are three schemes, and their costs are as follows:
Plan 1: Build 7 type a biogas digesters and 13 type B biogas digesters,
The total cost is: 7 × 2 + 13 × 3 = 530000 yuan 6 points
Plan 2: build 8 type a biogas digesters and 12 type B biogas digesters,
The total cost is: 8 × 2 + 12 × 3 = 520000 yuan 7 points
Plan 3: build 9 type a biogas digesters and 11 type B biogas digesters,
Total cost: 9 × 2 + 11 × 3 = 510000 yuan
The third plan is the most economical 8 points

A body with a mass of 2kg starts to move under the action of two common point forces. It is known that F1 = 3N and F2 = 4N. When the direction of F1 and F2 is opposite, the direction of F1 and F2 will be equal
If you want the resultant force to be 0, then you need to add a F3. How big is the F3 and where is the direction?

If F1 is perpendicular to F2, the resultant force of the two forces is 5N, and the acceleration is 5 / 2 = 2.5m/s ^ 2
If the resultant force is zero, F3 is also 5N, and the direction is opposite to that of F1 and F2

A multiplied by B = 60, then (a divided by 4) multiplied by (b multiplied by 4) equals () a + 2B = 68 (), 3A multiplied by B = (), a multiplied by (b divided by 2) = ()

(a divided by 4) multiplied by (b multiplied by 4) equals 60
3A times b = 180
A times (b divided by 2) = 30

Simple algorithm of 1 / 6 + 1 / 12 + 1 / 20 + 1 / 30 +. + 1 / 1999 * 2000

1/6+1/12+1/20+1/30+.+1/1999*2000
=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+…… +(1/1999-1/2000)
=1/2-1/2000
=999/2000

The number of even digits is prime, the number of ten digits is odd, and the number of two TENS is ()

It is known that f (x) is an odd function, and if x > 0, f (x) = x ^ 2 + 2x + 3, then X

x0
So - f (x) = f (- x) = (- x) & sup2; - 2x + 3 = x & sup2; - 2x + 3
Get X

How to fill in the fourth grade summer book
First find the rule and then fill in the number, by the way help me explain
Number: 252 1991 8448 747 12321 666
Yeah, volume two

The answer is: (14532)