Why is the three digit number composed of any three consecutive natural numbers from 1 to 9 a multiple of three

If the sum of the digits of a number can be divided by 3, then the number is a multiple of 3
Let the middle number be x, so the other two numbers are X-1 x + 1
The sum of the three digits = x + X-1 + X + 1 = 3x is a multiple of three
So the combination of three digits is also a multiple of 3

It is proved that if n + 1 numbers are taken out of the first 2n natural numbers, there must be two coprime numbers

The first 2n natural numbers 1,2,3,4,5,6 ,2n-1,2n
They were divided into n groups: (1,2), (3,4), (5,6) ,(2n-1,2n)
Take n + 1 numbers from the first 2n natural numbers (n groups), of which 2 numbers must belong to the same group,
That is, there must be two adjacent natural numbers
Because the greatest common divisor of two adjacent natural numbers is 1
So if we take n + 1 out of the first 2n natural numbers, there must be two coprime numbers

Drawer principle problem 1: any 9 natural numbers
Any 9 natural numbers, each with 8 to divide, there must be two of the same remainder, please explain the reason

This is very simple. If these nine numbers are divided by 8, the remainder can only be 1,2,3,4,5,6,7. Only these seven kinds of remainder have nine remainder, then there must be two of them

There are two resistors, R1 = 10 ohm and R2 = 15 ohm. What is the total resistance after they are connected in parallel?

R1*R2/[R1+R2]

How to calculate the simple algorithm of 13.7 × 0.28 × 8

13.7×0.28×8
=(12.5+1.2)*(0.25+0.03)*8
=12.5*0.25*8+12.5*0.03*8+1.2*0.25*8+1.2*0.03*8
=25+3+2.4+0.288
=30.688

Design experiment to measure resistance by voltammetry
Name... Principle... Equipment... Form
I didn't say. Is the design of experimental voltammetry resistance

Name: measuring resistance by voltammetry
Principle: according to Ohm's law, r = u / I
Equipment: one power supply, one switch, one adjustable resistor, one voltmeter, one ammeter, several connecting wires, and the measured resistance
Steps: 1. Circuit connection: the voltmeter is connected in parallel at both ends of the resistance to be measured. Then all devices are connected in series and then connected to the power supply
2. Close the switch
3. Record the reading U1 of voltmeter and I1 of ammeter
Adjust the variable resistor. Record U2 I2
Adjust the variable resistor again. Record U3 I3
Table: U I and U / I in various cases

3.8÷0.15=( )=( )÷15

3.8÷0.15
=(76 / 3)
=（380）÷15

What is the conversion rate between 1 pound and 1 kilogram
If you know,

1 lb = 0.45362451 (generally regarded as 0.4536) kg
According to the international measurement conversion standard, 1 pound = 453.6 grams, and 1 jin = 500 grams, so 1 pound = 0.972 Jin

The result of simplifying the quadratic radical a √ - A-1 / a ∧ 2 is
The answer should be - √ - A-1, but I don't know why,

The root sign (- A-1) / A & # 178; > = 0
So - A-1 > = 0
a

Xiao Wang's house is decorated. He goes to the store to buy light bulbs. At the counter of the store, there are 100 watt incandescent lamps and 40 Watt energy-saving lamps. Their unit prices are 2 yuan and 32 yuan respectively. It is known that the lighting effect and service life of these two kinds of lamps are the same. It is known that the electricity price of Xiao Wang's house is 0.5 yuan per kilowatt hour. When the service life of the two kinds of lamps exceeds how long, Xiao Wang will choose Energy saving lamp is cost effective? (electricity consumption (KWH) = power (kw) × time (H)

If the service life of these two kinds of lamps is more than 1000 hours, it is worthwhile for Xiao Wang to choose energy-saving lamps

Why is the three digit number composed of any three consecutive natural numbers from 1 to 9 a multiple of three