\(\dfrac{84}{20}=\dfrac{21}{5}\)

=42/10=21/5

\(\dfrac{84}{20}=\dfrac{84:4}{20:4}=\dfrac{21}{5}\)

Vì 35x=14y=10z
=> \(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{z}{7}\)

Áp dụng t/c của dãy tỉ số bằng nhau, ta có : 
\(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{z}{7}=\dfrac{\left(x+z-y\right)}{2+5-7}=\dfrac{20}{0}=0\)

Có : x/2 = 0 => x = 2*0 = 0
        y/5 = 0 => y = 5*0 = 0
        z/7 = 0 => z=7*0=0
Vậy, ..

\(15x=10y=6z\)

\(\Rightarrow\dfrac{x}{2}=\dfrac{y}{3}=\dfrac{z}{5}\)

Áp dụng t/c dtsbn:

\(\dfrac{x}{2}=\dfrac{y}{3}=\dfrac{z}{5}=\dfrac{x+y+z}{2+3+5}=\dfrac{20}{10}=2\)

\(\Rightarrow\left\{{}\begin{matrix}x=2.2=4\\y=2.3=6\\z=2.5=10\end{matrix}\right.\)

Đ 

S

Đ hay S ạ

C

C

C nhé

\(=\dfrac{1}{2}\left(\dfrac{2}{2\cdot3\cdot4}+\dfrac{2}{3\cdot4\cdot5}+...+\dfrac{2}{18\cdot19\cdot20}\right)\)

\(=\dfrac{1}{2}\left(\dfrac{1}{2\cdot3}-\dfrac{1}{3\cdot4}+\dfrac{1}{3\cdot4}-\dfrac{1}{4\cdot5}+...+\dfrac{1}{18\cdot19}-\dfrac{1}{19\cdot20}\right)\)

\(=\dfrac{1}{2}\left(\dfrac{1}{6}-\dfrac{1}{380}\right)\)

\(=\dfrac{187}{2280}\)

dề bài?

40 x 123 x 250 - 50 x 122 x 20

=(250x40)x123-122x(50x20)

=10000x123-122x100

=1230000-12200

=1217800

alo