Rational numbers are finite decimals All irrational numbers are infinitesimals Which is right and which is wrong? Please give me the reason

Rational numbers are finite decimals All irrational numbers are infinitesimals Which is right and which is wrong? Please give me the reason

Rational numbers are finite decimals
This sentence is wrong, because rational numbers can also be infinite circular decimals

Mathematical concepts in the first grade of junior high school in Shanghai
Shanghai. Nine year compulsory education that set of books
It's on the first day of junior high school
What is the concept of mathematics
There is also a way to solve the problem. "
Probably OK
Thank you~
Er. I want the whole concept

Rational number includes positive number, negative number and zero; integer includes monomial, polynomial and its operation; equation of one degree with one variable and its application problems; knowledge of basic graphics

Calculate 1 / 2 + 1 / 6 + 1 / 12 + 1 / 20 + 1 / 30 + 1 / 42 = 80 points mortgage!
Calculation 1 / 2 + 1 / 6 + 1 / 12 + 1 / 20 + 1 / 30 + 1 / 42=
Need explanation, analysis and formula, 80 points mortgage,

Method 1: very simple, 1 / 2 = 1-1 / 2
1/6=1/2-1/3
1/12=1/3-1/4
……
1/42=1/6-1/7
Add up to 1 / 2 1 / 2 - 1 / 7
That's 6 / 7
Method 2: 1 / 6 = 1 / 2-1 / 3
1/12=1/3-1/4
1/20=1/4-1/5
1/30=1/5-1/6
1/42=1/6-1/7
1/2+1/2-1/7=6/7
This is called split term summation
Method 3: 1 / 2 + 1 / 6 + 1 / 12 + 1 / 20 + 1 / 30 + 1 / 42
=1-1/2+1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-1/7
=6/7
Reference: if you have any questions, you can communicate with me. I'm very welcome,

What is cos θ divided by sin θ?
I know sin theta divided by cos theta equals Tan theta

Cos θ divided by sin θ equals cot θ, which is one third of Tan θ

A rectangular lawn is 12 meters long and 3 / 4 of its length wide. How many square meters is the area of this lawn?

Width: 12x3 / 4 = 9m
Area: 12x9 = 108m2
I wish learning progress, hope to adopt, do not know welcome to ask

13: What's the ratio of 39,

13:39=1:3
If you ask questions from your mobile phone, just click "satisfied" in the upper right corner of the client
Help each other, wish common progress!

We know the largest positive integer n of the arithmetic sequence {an}, the first term A1 > 0, a2011 + A2012 > 0, a2011 * a20120=
A：2011 B：2012 C：4023 D：4022
Which is the correct option

Solution
In the arithmetic sequence {an}
a1+a4022=a2+a4021=.=a2011+a2012>0
therefore
Sn4022>0
The answer is d
You can narrow the assumptions
Let A1 = 5, A2 = 2, A3 = - 1, A4 = - 4, A5 = - 7
We can see that A2 + a3 = 1 > 0, A2 * A30 holds the largest positive integer n = 4, not 5

Xiaomin's living room is 3 meters long and 2.5 meters wide. How many cm long square floor tiles do you choose? How many pieces do you need
To make a formula,

Take the common divisor of 3 and 2.5. Here you can take the square tiles with variable length of 50cm, then the length direction needs 3 / 0.5 = 6 tiles, the width direction needs 2.5 / 0.5 = 5 tiles, and the total number of tiles required is 6X5 = 30 tiles
Similarly, you can also take 25cm long square bricks, 3 / 0.25 = 12 in the length direction and 2.5 / 0.25 = 10 in the width direction, so a total of 12x10 = 120 will be required

The names, characteristics and distribution of 56 ethnic groups in China,

1. Achang: distributed in Yunnan, with a population of more than 27000, mainly engaged in agriculture, handicraft industry is also very developed, especially famous for being good at making long knives. It has its own language

Sin20 ° simplification

The use of sin60
According to the triple angle formula, sin 60 ° = 3 sin 20 ° - 4 Sin & sup 3; 20 °
Let sin20 ° = x, have √ 3 / 2 = 3x-4x & sup3
This is a univariate cubic equation, where a = 4, B = 0, C = - 3, d = √ 3 / 2, you can directly use the Kadang formula (the root formula of univariate cubic equation), you can get three solutions. Two of the three solutions are conjugate complex numbers, obviously rounding off, the rest is the solution you need
x=（－(q/2)-((q/2)²＋（p/3）³）^(1/2)）^(1/3)+（－(q/2)+((q/2)²＋（p/3）³）^(1/2)）^(1/3)
Where p = C / A, q = D / A, from Baidu Encyclopedia
You can put in the numbers