Tính 3./7 + (-5/2) + (-3/5)

a)= \(\dfrac{30}{70}+\left(-\dfrac{175}{70}\right)+\left(-\dfrac{42}{70}\right)\)

=\(-\dfrac{187}{70}\)

b) \(=-\dfrac{40}{30}+(-\dfrac{12}{30})+\left(-\dfrac{45}{30}\right)\)

\(=-\dfrac{97}{30}\)

\(\dfrac{2}{3}-4.\left(\dfrac{1}{2}\right)+\left(\dfrac{3}{4}\right)\)=

\(\dfrac{2}{3}-4.\left(\dfrac{1}{2}\right)+\left(\dfrac{3}{4}\right)\)

\(=\dfrac{2}{3}-2+\dfrac{3}{4}\)

=\(\dfrac{8}{12}-\dfrac{24}{12}+\dfrac{9}{12}\)

=\(\dfrac{-7}{12}\)

Tính :

C = \(-1-\dfrac{1}{3}-\dfrac{1}{6}-\dfrac{1}{10}-\dfrac{1}{15}-........-\dfrac{1}{1225}\)

Các bạn cho mình hỏi , với phép tính này đặt dấu trừ ra ngoài rồi bên trong đổi dấu , sau đó phải gấp 2 lần hay 1/2 ? Thầy mình dạy là 2 , còn mình thấy 1/2 mới đúng , vậy là làm sao ?

C = −(1+\(\dfrac{1}{3}+\dfrac{1}{6}\)+\(\dfrac{1}{10}+\dfrac{1}{15}\)+...+\(\dfrac{1}{1225}\))

\(\dfrac{1}{2}\)C =-\(\dfrac{1}{2}\)(1+\(\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{10}+...+\dfrac{1}{1225}\))

\(\dfrac{1}{2}C\) =-(\(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+...+\dfrac{1}{2450}\)

\(\dfrac{1}{2}\)C=-(\(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{49.50}\)

\(\dfrac{1}{2}\)C=-(1-\(\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}\)+....+\(\dfrac{1}{49}-\dfrac{1}{50}\))

\(\dfrac{1}{2}\)C=-(1-\(\dfrac{1}{50}\))

\(\dfrac{1}{2}\)C=-\(-\dfrac{49}{50}\)

C=\(-\dfrac{49}{50}:\dfrac{1}{2}=-\dfrac{49}{25}\)

Vậy C=-\(\dfrac{49}{25}\)

chúc bạn học tốt ạ