The school library bought 1000 story books, science and technology books and literature and art books. The science and technology books are 12 times more than the story books, and the literature and art books are 20 less than the story books How many story books, science and technology books and literature and art books did the school buy?

The school library bought 1000 story books, science and technology books and literature and art books. The science and technology books are 12 times more than the story books, and the literature and art books are 20 less than the story books How many story books, science and technology books and literature and art books did the school buy?

X + y + n = 10002x + 12 = Y N + 20 = x = x + y + n = 10002n + 40 + 12 = y 2n + 52 = Y20 + N + 2n + 52 + n = 1000 72 + 4N = 1000 n = 232 literature and art books: 232 story books: 232 + 20

There are 3600 science and technology books in the library, 20% more than story books. How many story books?

Story book = 3600 / (1 + 0.2) = 3000 (copies)

A student has an electric energy meter marked with "220 V 2.5 (10) 1500"
A classmate has an electric energy meter, which is marked with "220 V 2.5 (10) a 1500 R / kW · H". After connecting a light bulb with the electric energy meter in the home lighting circuit, he closed the switch, and the light bulb glowed normally. He found that the aluminum disc in the middle of the electric energy meter rotated 10 times in 4 minutes. Please calculate the energy consumption when the light bulb glowed normally for 1 hour?

Electric energy consumption is 0.1kw. H, 10 revolutions in 4 minutes, one hour is 10 * 15 = 150 revolutions, 1500 revolutions is one kilowatt hour, that 150 revolutions is 0.1 kilowatt hour, that is 0.1kw. H

-2007 power of 2 + 208 power of - 2
(- 2) 2007 power + (- 2) 2008 Power
How to calculate

2007 power of solution (- 2) = - 2 * 2 2006 Power (- 2) 2008 power = 4 * 2 2006 Power - 2 * 2 2006 power + 4 * 2 2006 power = 2 2007 power, the end of 1 power of 2 is 2; the end of 2 power is 4; the end of 3 power of 2 is 8; the end of 4 power of 2 is 6; the end of 5 power of 2 is 2 4...

Does the resistance of 220 V socket LED indicator play the role of voltage dividing or current limiting?

First of all, we need to know that the LED indicator is the light-emitting diode. The conduction voltage only needs 0.7V. The working current is only more than 10 mA. So to make it work normally and safely, we need to connect a resistor in series to divide the voltage and reduce the current. So we often choose about 200K resistor

The answers to the exercises on page 4 of the 1.1 fraction should be answered in 30 minutes. Wait, I'll write down one
3. When the letters in the following fraction meet what conditions, the fraction is meaningful?
Omit X-Y 1 / 2; 3a-b 2 / 2 A + BX 2 / 1

1. When a is not equal to 0, the fraction is meaningful
2. The fraction is meaningful when x is not equal to 1
3. When m is not equal to - 2 / 3, the fraction is meaningful
4. The fraction is meaningful when x is not equal to y
5. When a is not equal to 1 / 3b, the fraction is meaningful
6. The fraction is meaningful when x is not equal to plus or minus 1
Bonus points, no points, no motivation

The power is supplied to a lamp marked "PZ & nbsp; 220-60" in a distant construction site by a power supply of 220 v. due to the resistance of the wire, the actual power consumed by the bulb is 55 W. then the power consumed by the wire ()
A. Less than 5wb. Equal to 5wc. Greater than 5wd. Uncertain

If the wire resistance is ignored, that is, the voltage of 220 V is all connected to the bulb of "PZ & nbsp; 220-60", the bulb will light normally, and the power is 60 W; if the wire resistance is not ignored, that is, the resistance in the circuit will increase, and the total power consumed by the circuit will decrease, that is, less than 60 W, while the actual power of the bulb is 55 W, so the power on the wire can only be less than 5 W; therefore, select a

In the quadrilateral ABCD, the external angles of ∠ a, ∠ B and ∠ C are 30 °, 100 ° and 120 ° respectively. Calculate the degree of ∠ D (note the external angle)
In the quadrilateral ABCD, the external angles of ∠ a, ∠ B and ∠ C are 30 °, 100 ° and 120 ° respectively. Calculate the degree of ∠ D (note the external angle)

The external angles of ∠ a, ∠ B and ∠ C are 30 °, 100 ° and 120 ° respectively
Then, the external angles of a, B and C are 150 °, 80 ° and 60 ° respectively
However, the sum of ∠ a, B, C and D is 360 degrees, so ∠ d = 360-150-80-60 = 70 degrees

The open circuit voltage UOC of an active two terminal linear network is 6V and the short circuit current ISC is 2A. When the external resistance is 3W, the terminal voltage U is 0
A.3V
B.4V
C.6V

Your 3W should be 3 Euro? If it is 3 Euro, it should be a: 3V
When I = u / R is short circuited, I = 2, so the internal resistance of the power supply is 3 Ω. When the external resistance is 3 Ω, the series resistance of the whole circuit is 6 Ω, so I = 6 / 6 = 1a, u = IR, so u = 1 * 3 = 3V

1. The following chemical terms can express an element, an atom and a substance?
A.O B.Zn C.N2 D.Co
The answer is B. why
2. About SO2, CO2 and MnO2, the following statement is wrong
A. All of them contain oxygen B. all of them are oxides C. all of them are compounds D. all of them contain oxygen molecules
The answer is d. why is d wrong?
3. Alkali ()
4. the milk package of a certain emulsion company is described as follows:
Net content: 250G / box
Nutrients (per 100g)
Calcium ≥ 0.11g
Fat ≥ 3.3g
Protein ≥ 2.9g
If adults need at least 0.6g of calcium a day, and 90% of the calcium comes from milk, then a person should drink () boxes of this milk a day?
How to calculate

1B because Zn means zinc element, zinc atom, and elemental zinc (substance) 2D because the chemical term is like this, SO2, CO2, MnO2 originally means molecule, and it is impossible to have another oxygen molecule in it. It is correct to say that it contains oxygen element or two oxygen atoms. 3 Ca (oh) 2 calcium hydroxide is the solute of lime water, which is used to brush walls