Question

[Nguyễn Đức Huy]

Làm giúp mik nhưng bài này vs:

1/Viết các số thập phân sau thành phân số:

a) 2,35;b) 1,(18);c) 1,2(93)

2/Tính

a) A=2\(\dfrac{1}{3}\)+(-0,25) + \(\dfrac{3}{8}\)-2,15+\(\dfrac{1}{12}\)/

b) B=\(\dfrac{1}{1.3}\)+\(\dfrac{1}{3.5}\)+\(\dfrac{1}{5.7}\)+...+\(\dfrac{1}{29.31}\)

Answer 1

[Đan Anh]

B)

B = \(\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+...+\dfrac{1}{29.31}\)

= \(\dfrac{1}{2}.\left(\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+...\dfrac{1}{29.31}\right)\)

= \(\dfrac{1}{2}.\left(\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{29}-\dfrac{1}{31}\right)\)

= \(\dfrac{1}{2}.\left(\dfrac{1}{1}-\dfrac{1}{31}\right)\)

= \(\dfrac{1}{2}.\dfrac{30}{31}\)

= \(\dfrac{30}{62}\) = \(\dfrac{15}{31}\)

Answer 2

[Nguyễn Ngọc Linh Châu]

1A,47/20

B.13/11

C,427/330

2A,47/120

B,15/31