Vì không khí có hơi nước, mà hơi nước có độ ẩm nên không khí có độ ẩm

\(c=\dfrac{1}{1.2.3.4}+\dfrac{1}{2.3.4.5}+...+\dfrac{1}{27.28.29.30}\)

\(3C=\dfrac{1}{1.2.3}-\dfrac{1}{2.3.4}+\dfrac{1}{2.3.4}+...+\dfrac{1}{27.28.29}-\dfrac{1}{28.29.30}\)

\(c=\dfrac{1}{1.2.3}-\dfrac{1}{28.29.30}\)\(=\)\(\dfrac{1}{6}-\dfrac{1}{24360}\)

\(C=\) \(\dfrac{4059}{24360}\)

Hello everybody. My name is Phuong. I am in class 6A and study at Song Tri secondary school. I live Ky Anh town. Today, I am going to tell you about my best friend

My best friend's full name is Nguyen Tran Thao Trang. She is in class 6A and study at Ky Trinh secondary school. She is very . She has a round face and she has long black hair. Her eyes is black glitter as two droplets water. She likes an ice-cream when it's hot and play volleyball. Her favorite subject is math, because she learns very good math. She often goes swimming with me in the summer and after we together go eat an ice-cream. I love Trang

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the following questions.

For lunch, I always haave something quick and easy: a sandwich, a salad, toast and the _______.

A. same

B. similar

C. like

D. rest

2. b) \(5A=1-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{11}+...+\dfrac{1}{496}-\dfrac{1}{501}\)

\(=1-\dfrac{1}{501}=\dfrac{500}{501}\)

\(\Rightarrow A=\dfrac{100}{501}\)

1. tính nhanh:

A \(=\dfrac{1}{3}+\dfrac{1}{3^2}+...+\dfrac{1}{3^8}\)

3A = \(1+\dfrac{1}{3}+\dfrac{1}{3^2}+...+\dfrac{1}{3^7}\) (1)

A = \(\dfrac{1}{3}+\dfrac{1}{3^2}+...+\dfrac{1}{3^7}+\dfrac{1}{3^8}\) (2)

Lay (1)-(2) ta duoc:

\(2A=1-\dfrac{1}{3^8}=1-\dfrac{1}{6561}=\dfrac{6560}{6561}\)

\(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\)...\(+\dfrac{1}{100.101}\)

\(=\left(1-\dfrac{1}{2}\right)+\left(\dfrac{1}{2}-\dfrac{1}{3}\right)+...+\left(\dfrac{1}{100}-\dfrac{1}{101}\right)\)

\(=1-\dfrac{1}{101}=\dfrac{100}{101}\)