1 + 3 + 5 + 7 +. + (2n-1) how to find the number of terms in this sequence

1 + 3 + 5 + 7 +. + (2n-1) how to find the number of terms in this sequence

This is an arithmetic sequence with a tolerance of 2
Number of items = (last first) / tolerance + 1 = (2n-1-1) / 2 + 1 = n-1 + 1 = n
Hope to help you

The arithmetic sequence {an}, the number of terms is 2n, why s odd / s even = (an + 1) / an?

S odd = a1 + a3 + A5 + +A (2n-3) + a (2n-1) s couple = A2 + A4 + A6 + +A (2n-2) + A2N if n is odd, a1 + a (2n-1) = A3 + a (2n-3) = =A(n-2)+A(n+2)=2AnA2+A2n=A4+A(2n-2)=…… =A (n-1) + a (n + 3) = 2A (n + 1) s odd = NANs even = Na (n + 1) s odd / s even = an / a (n + 1) if

An arithmetic sequence has a total of 2N-1 terms. If the sum of the items of the sequence is 2008 and an = 8, find the number of terms n

S(2n-1)
=[a1+a(2n-1)]*(2n-1)/2
1+(2n-1)=n+n
So a1 + a (2n + 1) = an + an = 2An
That is, Sn = 2An * (2n-1) / 2 = 2008
2n-1=2008/an=251
n=126

Mr. Shi's speed of marking mathematics homework is twice as fast as that of mathematics class, two classes 120
How long does it take for teachers and class representatives to read homework history at the same time?

Mathematics class can be approved M / N in one minute, history teacher can approve 2m / N, and the time is 120 / (2m / N + m / N) = 40n / m at the same time
minute

I'm the representative of math class. The math teacher asked me to write a reflection report
It's just about the number of words in mathematics

Dear teacher: Hello, because some time ago I was more lax in the study of mathematics subjects, resulting in the mathematics test did not test well, I analyzed all aspects of mathematics learning, I think there are the following reasons: first, I did not seriously preview before class, there is no preliminary understanding of what the teacher wants to say

On the first day of September 1, the representative of the fifth grade mathematics class reported to Mr. Li: "Mr. Li, our 100 students have done a total of 1600 math problems in the summer vacation." Mr. Li was very happy to hear that, and immediately praised them, and said: "among you 100 people, at least four of you have done as many math problems." is this sentence right? Why?

What Miss Li said is right
Let's divide the 100 students in grade five into three groups, and there is one person left in 33 groups. Suppose three people in the first group didn't do the problem, that is, everyone did 0 problem; everyone in the second group did 1 problem; everyone in the third group did 2 problems In this way, everyone in group 33 did 32 questions. If the remaining one did not have the same number of questions as the 99 people in front of him, he would have to do at least 33 questions
3×（0+1+2+3+…… +31 + 32) + 33 = 1617
More than 1600 questions. No more than 1600 questions, there must be one or more students who do less questions. In fact, as long as there is one student who does less questions, then this student can be assigned to the group who does less questions. In this way, four students in that group will do the same number of questions. That is to say, among the 100 students, at least four students will do the same number of math problems

Zhao, Qian, and sun are teachers of mathematics, Chinese, and foreign languages respectively. It is known that each teacher teaches only one course. In addition, there are some situations as follows: ① Mr. Zhao uses Chinese in all classes. ② the foreign language teacher is a student's brother. ③ Mr. Sun is a woman. She asked the math teacher a question. Which course do the three teachers teach

From (1) we can see that Mr. Zhao is not an English teacher, because the foreign language teacher is a student's brother, so the foreign language teacher is a man, because Mr. Sun is a woman, so Mr. Sun can not be a foreign language teacher, so the foreign language teacher can only be Mr. Qian; and because Mr. Sun is a woman, she asked a question to the math teacher, so Mr. Sun is not a math teacher, she can only be a teacher Chinese teacher, so Mr. Zhao is a math teacher, so Mr. Sun is a Chinese teacher, Mr. Zhao is a math teacher, and Mr. Qian is a foreign language teacher. A: Mr. Sun is a Chinese teacher, Mr. Zhao is a math teacher, and Mr. Qian is a foreign language teacher

Wang, Liu and Li are teachers of Chinese, mathematics and English respectively. Each of them teaches only one course: A. Wang teaches Chinese in class, B,
English teacher is a student's brother; C, Li is a female, she asked a question to the math teacher; can you judge which course these three teachers teach?

Mr. Wang is not an English teacher because he uses Chinese in class
Because the English teacher is a student's brother, and Miss Li is a woman, so Miss Li is not an English teacher
So the English teacher can only be Miss Liu
And because Mr. Li asked the math teacher a question, so Mr. Li can't be a math teacher
And the math teacher can only be Mr. Wang
So Miss Li is a Chinese teacher

25. Teachers a, B and C are responsible for the teaching of Chinese, mathematics, English, music, physical education and fine arts in class 6 (2), with each teacher teaching

A teaches Chinese and art
B teach math, P.E
C teaches English and music

Zhang Cong, Wang Ren and Chen Ming are responsible for the teaching of Chinese, mathematics, English, music, art and physical education in class 52
Make the language concise
Zhang Cong, Wang Ren and Chen Ming performed the teaching of Chinese, mathematics, English, music, fine arts and physical education in class 52, two for each. Now we know: 1. English teacher and math teacher are neighbors; 2. Wang Ren is the youngest; 3. Zhang Cong likes to communicate with PE teacher and math teacher; 4. PE teacher is older than Chinese teacher; 5. Wang Ren, Chinese teacher and music teacher often do exercises together. Please judge which two courses they teach respectively.

According to 1: English and mathematics are not the same person
2: Wang Ren doesn't teach sports
3: Zhang Cong does not teach mathematics and physical education, and mathematics and physical education are not the same person
4: Wang Ren doesn't teach sports
5: Wang Ren doesn't teach Chinese and music. And Chinese and music are not the same person
According to 1,4,5, Wang Renjiao is one of art and English mathematics
Suppose that Wang Ren's other subject is English
Zhang Cong teaches Chinese and music, contrary to 5
Wang Ren teaches art and mathematics
According to 3,5
Zhang Cong teaches Chinese and English
Chen Ming teaches music and sports
Note: the general method of this question is to analyze the implied condition of each piece of information first, and then to analyze the contradiction and draw a conclusion