\(A=\dfrac{-5x}{21}+\dfrac{-5y}{21}+\dfrac{-5z}{21}\)
\(A=\dfrac{-5x+\left(-5y\right)}{21}+\dfrac{-5z}{21}\)
\(A=\dfrac{-5\cdot\left(x+y\right)}{21}+\dfrac{-5z}{21}\)
\(A=\dfrac{-5\cdot\left(-z\right)}{21}+\dfrac{-5z}{21}\)
\(A=\dfrac{5z}{21}+\dfrac{-5z}{21}\)
\(A=\dfrac{5z+\left(-5z\right)}{21}=\dfrac{0}{21}=0\)
Vậy \(A=0\)
Tìm x,y,z
\(\dfrac{-5x}{21}+\dfrac{-5y}{21}+\dfrac{-5z}{21}\) và z+y=-Z
Tính giá trị biểu thức
A= -5x/21 + 5y/21 + -5z/21 biết x + y = -z
Tìm x thuộc Z
A=\(\dfrac{5x}{3}\):\(\dfrac{10x+5x}{21}\)
tính theo cách hợp lý
a)\(\dfrac{4}{20}+\dfrac{16}{42}+\dfrac{6}{15}+\dfrac{-3}{5}+\dfrac{2}{21}+\dfrac{-40}{21}+\dfrac{3}{20}\)
B)\(\dfrac{42}{46}+\dfrac{250}{186}+\dfrac{-2121}{2323}+\dfrac{-125125}{143143}\)
Chứng minh rằng :
\(\dfrac{7}{12}< \dfrac{1}{21}+\dfrac{1}{20}+...+\dfrac{1}{40}< \dfrac{5}{6}\)
( Chú ý : \(\dfrac{1}{21}+\dfrac{1}{20}\)chứ k phải \(\dfrac{1}{21}+\dfrac{1}{22}\) nha )
\(\dfrac{-5}{21}\)+\(\dfrac{-2}{21}\)+\(\dfrac{8}{24}\)
Tìm x biết :
a) 21 + 3 . x = 39
b) 2x - \(\dfrac{1}{2}\)= \(\dfrac{-9}{10} \): 0,2
Tìm x
a. \(x-\dfrac{20}{11.13}-\dfrac{20}{13.15}-\dfrac{20}{15.17}-...-\dfrac{20}{53.55}\)
b. \(\dfrac{1}{21}+\dfrac{1}{28}+\dfrac{1}{36}+...+\dfrac{2}{x\left(x+1\right)}=\dfrac{2}{9}\)
Tìm n \(\in\) N* biết \(\dfrac{1}{21}+\dfrac{1}{77}+\dfrac{1}{165}+...+\dfrac{1}{n^2+4n}=\dfrac{56}{673}\)
