= 
\(\frac{-10z}{21} + \frac{-5z}{21} \) = \(\frac{-15z}{21} \)
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Ta có :
A = \(\frac{-5.x}{21}+\frac{-5.y}{21}+\frac{-5.z}{21}\)
= \(\frac{-5}{21}.\left(x+y+z\right)\)
= \(\frac{-5}{21}.\left(-z+z\right)\)
= \(\frac{-5}{21}.0\)
= 0
Vậy A = 0
A=-5(x+y+z)/21
A=-5(-z+z)/21
A=-5x0/21
A=0
có x+y=-z
=> x+y+z=0
=> A = \(\frac{-5x}{21}+\frac{-5y}{21}+\frac{-5z}{21}=\frac{-5\left(x+y+z\right)}{21}=0\)
vậy .....
hok tốt
Ta có: \(A=\frac{-5x}{21}+\frac{-5y}{21}+\frac{-5z}{21}\)
\(\Leftrightarrow A=\frac{-5}{21}\left(x+y+z\right)\)
Mà\(x+y=-z\)
\(\Rightarrow A=\frac{-5}{21}\left(-z+z\right)\)
\(\Leftrightarrow A=\frac{-5}{21}.0\)
\(\Leftrightarrow A=0\)
Vậy\(A=0\)
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