Question

bài 1:Tính nhanh

\(\frac{1}{3.4}+\frac{1}{4.5}+.....+\frac{1}{89.90}\)

 

 

Answer 1

Ta có : \(\frac{1}{3.4}+\frac{1}{4.5}+......+\frac{1}{89.90}\)

\(=\frac{1}{3}+\frac{1}{4}-\frac{1}{4}+.......+\frac{1}{89}-\frac{1}{90}\)

\(=\frac{1}{3}-\frac{1}{90}=\frac{30}{90}-\frac{1}{90}=\frac{29}{90}\)

Answer 2

\(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{89.90}\)

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{89}-\frac{1}{90}\)

\(=\frac{1}{3}-\frac{1}{90}\)

\(=\frac{29}{90}\)

Answer 3

Đặt \(A=\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{89.90}\)

\(A=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{89}-\frac{1}{90}\)

\(A=\frac{1}{3}-\frac{1}{90}\)

\(A=\frac{29}{90}\)

Answer 4

\(\frac{1}{3.4}+\frac{1}{4.5}+.....+\frac{1}{89.90}\)

= \(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+.....+\frac{1}{89}-\frac{1}{90}\)

= \(\frac{1}{3}-\frac{1}{90}\)

= \(\frac{29}{90}\)

Answer 5

\(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{89.90}\)

\(=\frac{4-3}{3\cdot4}+\frac{5-4}{4\cdot5}+...+\frac{90-89}{89\cdot90}\)

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{89}-\frac{1}{90}\)

\(=\frac{1}{3}-\frac{1}{90}=\frac{29}{90}\)

Answer 6

\(=\frac{29}{90}\)nha bạn

Answer 7

1/3.4 + 1/4.5 + ... + 1/89.90

= 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/89 - 1/90

= 1/3 - 1/90

= 29/90

Đs:

Ps: Sao câu này có nhiều ctv trả lời thế nhỉ?