In high school mathematics definite integral What does DX in F (x) DX stand for? How to calculate between DX?

In high school mathematics definite integral What does DX in F (x) DX stand for? How to calculate between DX?

Because the intuitive meaning of definite integral is the area around the curve and X axis, DX can be regarded as the bottom edge of this small square. From the perspective of real infinity, when DX is infinitesimal, we can understand the height of this small square as f (x). From the perspective of potential infinity, the area of such a trapezoid with curved edge follows DX as the bottom edge, If f (x) is a high rectangle, the difference of its area can always be less than any positive real number. So their product f (x) DX is the differential dy of Y. since it is a differential, of course, it satisfies the differential algorithm

If the voltage at both ends of AB is u and the reading ratio of ammeter is 1 after the switch is opened and closed, then the resistance ratio of two resistors is 1

4: The current ratio is inversely proportional to the resistance ratio

Finding the number of integer solutions of equation | X-2 | + | x + 1 | = 3
Huanggang secret volume seven. Secret volume practice test 16, question 25,

When x is greater than or equal to 2, the equation = X-2 + X + 1 = 3, that is, 2x = 4, x = 2
When x is less than 2 and greater than or equal to - 1, the equation = 2-x + X + 1 = 3, that is, 3 = 3 is an identity
The integer solution is 1, 0, - 1
When x is less than - 1, the equation = 2-x-x-1 = 3, that is - 2x = 2, x = - 1
So the integer solution is - 1,0,1,2

What is the unit of conductivity? The relationship between conductivity and resistance
We know that the resistance is 16-18m Ω / cm,
What is the conductivity?
I want to know the answer

http://blog.sina.com.cn/u/1233721385 Conductivity is the ability of an object to conduct current. The measuring principle of conductivity meter is to put two parallel plates into the solution to be measured, add a certain potential (usually sine wave voltage) to both ends of the plates, and then measure the current flowing between the plates

[x - (2x of X + 1)] X & # 178; - 1 x & # 178; - 2x + 1

[x - (2x of X + 1)] X & # 178; - 1 x & # 178; - 2x + 1
＝[x-(2x/x+1)]×[(x+1)(x-1)/(x-1)²]
＝[(x²-x)/(x+1)]×[(x+1)/(x-1)]
＝[x(x-1)/(x+1)]×[(x+1)/(x-1)]
＝x

The power of several electrical appliances in the figure is closest to 1000W ()
A. Home fan B. laptop C. desk lamp D. rice cooker

A. The rated power of electric fan is about 60W, so a is wrong; B, the power of laptop is about 100W, so B is wrong; C, the rated power of desk lamp is about 40W, so C is wrong; D, the power of electric cooker is about 1000W, so D is correct

389961 △ 39 (simple calculation)
389961÷39
This is a simple calculation,

389961÷39
=(389961+39-39)÷39
=(390000-39)÷39
=390000÷39-39÷39
=10000-1
=9999

A 220 V, 25 W incandescent lamp is connected to the 220 V line. How much is the passing current? How much is the bulb resistance?

Current I = P / u = 25 / 220 = 0.114 (a)
Resistance R = u * U / P = 220 * 220 / 25 = 1936 (Ohm)
This is only in DC state value, AC value is close to indication value

Simple calculation 5.8 * 101-5.8

5.8*101-5.8
=5.8*（101-1）
=5.8*100
=580

Connect the resistor with R1 equal to 5 ohm and R2 equal to 10 ohm into the circuit. If the current in the circuit is 1.2A, calculate their total resistance, two voltage segments of each resistor and the power supply voltage

If it is connected in series, the total resistance is 15 Ω, and the power supply voltage is u = 1.2 * 15 = 18V; the voltages of the two sections of the two resistors are 1.2 * 5 = 6 and 1.5 * 10 = 15 respectively;
When it is connected in parallel, the total resistance is (10 * 5) / (10 + 5) = 10 / 3 Ω, the power supply voltage is u = 1.2 * (10 / 3) = 4V, and the voltage of both sections is 4V;