Question

[Lê Thị Trà MI]

tìm x,y,z,t thuộc N tỏa mãn : 31 ( xyzt + xy + xt + zt + 1 ) = 40 ( yzt + y + t )

Answer 1

[Le Thi Khanh Huyen]

Ta có :

\(31\left(xyzt+xy+xt+zt+1\right)=40\left(yzt+y+t\right)\)

\(\Rightarrow\frac{xyzt+xy+xt+zt+1}{yzt+y+t}=\frac{40}{31}\)

\(\Rightarrow\frac{x\left(yzt+y+t\right)+zt+1}{yzt+y+t}=\frac{40}{31}\)

\(\Rightarrow x+\frac{zt+1}{yzt+y+t}=\frac{40}{31}\)

\(\Rightarrow x+\frac{1}{\left(\frac{yzt+y+t}{zt+1}\right)}=\frac{40}{31}\)

\(\Rightarrow x+\frac{1}{\left(y+\frac{t}{zt+1}\right)}=\frac{40}{31}\)

\(\Rightarrow x+\frac{1}{y+\frac{1}{\left(\frac{zt+1}{t}\right)}}=\frac{40}{31}\)

\(\Rightarrow x+\frac{1}{y+\frac{1}{z+\frac{1}{t}}}=\frac{40}{31}\)

\(\frac{40}{31}< \frac{62}{31}=2\Rightarrow x< 2\)

Với x = 0; có :

\(\frac{1}{y+\frac{1}{z+\frac{1}{t}}}=\frac{40}{31}\)

\(\Rightarrow y+\frac{1}{z+\frac{1}{t}}=\frac{31}{40}\)

Mà \(\frac{31}{40}< 1\Rightarrow y< 1\Rightarrow y=0\)

\(\Rightarrow\frac{1}{z+\frac{1}{t}}=\frac{31}{40}\)

\(\Rightarrow z+\frac{1}{t}=\frac{40}{31}\)

\(\cdot z=0\Rightarrow t=\frac{31}{40}\notin Z\)(Loại )

\(\cdot z=1\Rightarrow t=\frac{31}{9}\notin Z\)(Loại )

Với \(x=1;\)ta có :

\(\frac{1}{y+\frac{1}{z+\frac{1}{t}}}=\frac{40}{31}-1\)

\(\Rightarrow\frac{1}{y+\frac{1}{z+\frac{1}{t}}}=\frac{9}{31}\)

\(\Rightarrow y+\frac{1}{z+\frac{1}{t}}=\frac{31}{9}\)

\(\frac{31}{9}< \frac{36}{9}=4\Rightarrow y< 4\)

\(\cdot y=0\Rightarrow z+\frac{1}{t}=\frac{9}{31}\Rightarrow z=0\Rightarrow t=\frac{31}{9}\notin Z\)(Loại)

\(\cdot y=1\Rightarrow z+\frac{1}{t}=\frac{9}{22}\Rightarrow z=0\Rightarrow t=\frac{22}{9}\notin Z\)(Loại)

\(\cdot y=2\Rightarrow z+\frac{1}{t}=\frac{9}{13}\Rightarrow z=0\Rightarrow t=\frac{13}{9}\notin Z\)(Loại )

\(\cdot y=3\Rightarrow z+\frac{1}{t}=\frac{9}{4}\)

\(\frac{9}{4}< 3\Rightarrow z< 3\)

1. \(z=0\Rightarrow t=\frac{4}{9}\notin Z\)
2. \(z=1\Rightarrow t=\frac{4}{5}\notin Z\)
3. \(z=2\Rightarrow t=4\)( Thỏa mãn )

Vậy \(x=1;y=3;z=2;t=4.\)