x=6;y=14 nha bạn

xy+3x-7y = 21

x.(y+3)- 7y-21 = 0

x.(y+3)- 7.(y+3) =0

(x-7).(y+3) =0

=> x-7 = 0 hoặc y+3 = o

TH 1: x-7 = 0 => x = 7

TH2: x+3 = 0 => x= -3

Ta co:x.y+3.x-7.y=21

=>x.(y+3)-7.y=21

=>x.(y+3)-7.y-21=0

=>x.(y+3)-(7.y+21)=0

=>x.(y+3)-7.(y+3)=0

=>=>(x-7).(y+3)=0

=>x-7=0 hoac y+3=0

=>x=7 hoac y=(-3)

Vay voi x=7 thi y thoa man moi gia tri(y thuoc Z);y=(-3) thi x thoa man moi gia tri(x thuoc Z

Theo đề, ta có:  \(\dfrac{1+2x}{18}=\dfrac{1+4x}{34}\)

\(\Leftrightarrow34\left(1+2x\right)=18\left(1+4x\right)\)

\(\Leftrightarrow34+68x=18+72x\)

\(\Leftrightarrow34-18=72x-68x\)

\(\Leftrightarrow16=4x\)

\(\Leftrightarrow x=4\)

Khi \(x=4\) vào ta có:   \(\dfrac{1+4.4}{34}=\dfrac{1+6.4}{2y^2}\Leftrightarrow\dfrac{1}{2}=\dfrac{25}{2y^2}\) 

\(\Leftrightarrow2y^2=50\)

\(\Leftrightarrow y^2=50\)

\(\Leftrightarrow y=\pm5\)

Ta có: (2 - x)(4/5 - x) < 0

=> \(\hept{\begin{cases}2-x>0\\\frac{4}{5}-x< 0\end{cases}}\) hoặc \(\hept{\begin{cases}2-x< 0\\\frac{4}{5}-x>0\end{cases}}\)

=> \(\hept{\begin{cases}x>2\\x< \frac{4}{5}\end{cases}}\) (loại) hoặc \(\hept{\begin{cases}x< 2\\x>\frac{4}{5}\end{cases}}\)

=> \(\frac{4}{5}< x< 2\)

\(\left(2-x\right)\left(\frac{4}{5}-x\right)< 0\)

TH1 : \(\hept{\begin{cases}2-x>0\\\frac{4}{5}-x< 0\end{cases}\Rightarrow\hept{\begin{cases}2>x\\\frac{4}{5}< x\end{cases}}}\)\(\Rightarrow\frac{4}{5}< x< 2\)

Th2 : \(\hept{\begin{cases}2-x< 0\\\frac{4}{5}-x>0\end{cases}\Rightarrow\hept{\begin{cases}2< x\\\frac{4}{5}>x\end{cases}}}\)\(\Rightarrow x\in\varnothing\)

Vậy \(\frac{4}{5}< x< 2\)

cảm ơn các bạn nhiều, kết bạn với nhau đi,