y x 10 + y + 10 = 200

y x ( 10 + 10 )   = 200

y x 20               = 200

y                      = 200 : 20

y                      = 10

y x 10 + y x 10 = 200

y x ( 10 + 10 ) = 200

y x 20 = 200

y = 200 : 20

y = 10 

( y+ y ) . 10 = 200

y.2.10=200

y . 20 = 200

y =200 / 20

y = 10

Lời giải:
$y\times 10\times 0,41+y\times 5,9=200$

$y\times 4,1+y\times 5,9=200$

$y\times (4,1+5,9)=200$

$y\times 10=200$

$y=200:10=20$

=> y x ( 4 + 6 ) = 20

=> y x 10 =20

=> y = 2

y x 4 + y x 6 = 200 : 10

y x ( 4 + 6 ) = 20

y x 10 = 20

y = 20 : 10

y = 2

y x 4 + y x 6 = 200 : 10

y x ( 4 + 6 ) = 20

y x 10 = 20

       y  = 20 : 10

       y  = 2                           k mk nha~~

a) Ta có: \(\dfrac{x}{y}=\dfrac{10}{9}\Rightarrow\dfrac{x}{10}=\dfrac{y}{9}\)

               \(\dfrac{y}{z}=\dfrac{3}{4}\Rightarrow\dfrac{y}{3}=\dfrac{z}{4}\Rightarrow\dfrac{y}{9}=\dfrac{z}{12}\)

\(\Rightarrow\dfrac{x}{10}=\dfrac{y}{9}=\dfrac{z}{12}=\dfrac{x-y+z}{10-9+12}=\dfrac{78}{13}=6\)

\(\Rightarrow\left\{{}\begin{matrix}x=6.10=60\\y=6.9=54\\z=6.12=72\end{matrix}\right.\)

b)Ta có:  \(\dfrac{x}{y}=\dfrac{9}{7}\Rightarrow\dfrac{x}{9}=\dfrac{y}{7}\)

               \(\dfrac{y}{z}=\dfrac{7}{3}\Rightarrow\dfrac{y}{7}=\dfrac{z}{3}\)

\(\Rightarrow\dfrac{x}{9}=\dfrac{y}{7}=\dfrac{z}{3}=\dfrac{x-y+z}{9-7+3}=-\dfrac{15}{5}=-3\)

\(\Rightarrow\left\{{}\begin{matrix}x=-3.9=-27\\y=-3.7=-21\\z=-3.3=-9\end{matrix}\right.\)

c) \(\dfrac{x}{3}=\dfrac{y}{4}=\dfrac{z}{3}\)

\(\Rightarrow\dfrac{x^2}{9}=\dfrac{y^2}{16}=\dfrac{z^2}{9}=\dfrac{x^2+y^2+z^2}{9+16+9}=\dfrac{200}{34}=\dfrac{100}{17}\)

\(\Rightarrow\left\{{}\begin{matrix}x^2=\dfrac{900}{17}\\y^2=\dfrac{1600}{17}\\z^2=\dfrac{900}{17}\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}x=\pm\dfrac{30\sqrt{17}}{17}\\y=\pm\dfrac{40\sqrt{17}}{17}\\z=\pm\dfrac{30\sqrt{17}}{17}\end{matrix}\right.\)

Vậy\(\left(x;y;z\right)\in\left\{\left(\dfrac{30\sqrt{17}}{17};\dfrac{40\sqrt{17}}{17};\dfrac{30\sqrt{17}}{17}\right),\left(-\dfrac{30\sqrt{17}}{17};-\dfrac{40\sqrt{17}}{17};-\dfrac{30\sqrt{17}}{17}\right)\right\}\)

 mik mới hk lop 5

B nha em

Chọn B

 x + y - 200 

Thay số : 15 + 10 - 200

= 25 - 200

= -175

thay x=15 và y=10 vào biểu thức, ta có:

15+10-200=-175

Tại x=15;y=10 thay vào biểu thức ta có:

x+y-200=15+10-200

            =25-200

             =-175