\(x:1=y:2=\frac{x}{1}=\frac{y}{2}=\frac{x^2}{1}=\frac{y^2}{4}\) và x²+y²=20

Ap dụng tính chất dãy tỉ số bằng nhau : 

\(\frac{x^2}{1}=\frac{y^2}{4}=\frac{x^2+y^2}{1+4}=\frac{20}{5}=4\)

Suy ra : 

\(\frac{x^2}{1}=4\Rightarrow x^2=4.1=4\Rightarrow x=2\Rightarrow x=\)+2

\(\frac{y^2}{4}=4\Rightarrow y^2=4.4=16\Rightarrow y=4\Rightarrow y=\)+4

Khi x+y=-2+-4=-6 (ko lay 2+4 vì đề cho x;y<0)

Vay x+y=-6

\(x:1=y:2\Rightarrow\frac{x}{1}=\frac{y}{2}\Rightarrow\frac{x^2}{1}=\frac{y^2}{4}\)

Theo t/c dãy TSBN:

\(\frac{x^2}{1}=\frac{y^2}{4}=\frac{x^2+y^2}{1+4}=\frac{20}{5}=4\)

=> \(\frac{x^2}{1}=4\Rightarrow x^2=4=2^2=\left(-2\right)^2\Rightarrow x=\)+2

=> \(\frac{y^2}{4}=4\Rightarrow y^2=4.4=16=4^2=\left(-4\right)^2\Rightarrow y=\)+4

Mà x;y < 0

=> x = -2; y=-4

Vậy x + y = -2 + (-4) = -6.

số thập phân là 34,69

số tự nhiên là 1045

2)

a) Ta có:

\(n+3⋮n-1\)

\(\Rightarrow\left(n-1\right)+4⋮n-1\)

\(\Rightarrow4⋮n-1\)

\(\Rightarrow n-1\in U\left(4\right)=\left\{1;2;4\right\}\) ( Vì \(n\in N\) )

\(\Rightarrow\left\{{}\begin{matrix}n-1=1\Rightarrow n=2\\n-1=2\Rightarrow n=3\\n-1=4\Rightarrow n=5\end{matrix}\right.\)

Vậy \(n\in\left\{2;3;5\right\}\)

b) Ta có:

\(4n+3⋮2n+1\)

\(\Rightarrow\left(4n+2\right)+1⋮2n+1\)

\(\Rightarrow2\left(2n+1\right)+1⋮2n+1\)

\(\Rightarrow1⋮2n+1\)

\(\Rightarrow2n+1\in U\left(1\right)=\left\{1\right\}\) ( Vì \(n\in N\) )

\(\Rightarrow2n+1=1\)

\(\Rightarrow n=0\)

Vậy \(n=0\)

72

(Sorry nha, mình không có lời giải)

K=4,5

Nho tick nha

Tick di roi day trinh bay

dư 1 

cách tính trong chtt nhưng đây đề khác dư 1 vì +1 

các bạn cho mk vài li-ke cho tròn 720 với 

\(y=f\left(x\right)=\dfrac{5}{x-1}\)

a) ĐKXĐ khi: \(x-1\ne0\) \(\Leftrightarrow x\ne1\)

b)

\(y=f\left(-2\right)=\dfrac{5}{-2-1}=\dfrac{5}{-\left(2+1\right)}=\dfrac{5}{-3}=\dfrac{-5}{3}\)

\(y=f\left(\dfrac{1}{3}\right)=\dfrac{5}{\dfrac{1}{3}-1}=\dfrac{5}{\dfrac{1}{3}-\dfrac{3}{3}}=\dfrac{5}{\dfrac{-2}{3}}=\dfrac{-15}{2}\)

c)

* \(y=f\left(x\right)=\dfrac{5}{x-1}=-1\)

\(\Rightarrow x-1=5:\left(-1\right)\)

\(\Rightarrow x-1=-5\)

\(\Rightarrow x=-5+1\)

\(\Rightarrow x=-\left(5-1\right)\)

\(\Rightarrow x=-4\)

Vậy \(y=-1\) thì \(x=-4\)

* \(y=f\left(x\right)=\dfrac{5}{x-1}=1\)

\(\Rightarrow x-1=5:1\)

\(\Rightarrow x-1=5\)

\(\Rightarrow x=5+1\)

\(\Rightarrow x=6\)

Vậy \(y=1\) thì \(x=6\)

* \(y=f\left(x\right)=\dfrac{5}{x-1}=\dfrac{1}{5}\)

\(\Rightarrow x-1=5:\dfrac{1}{5}\)

\(\Rightarrow x-1=5.5\)

\(\Rightarrow x-1=25\)

\(\Rightarrow x=25+1\)

\(\Rightarrow x=26\)

Vậy \(y=\dfrac{1}{5}\) thì \(x=26\)

\(\frac{x}{3}=\frac{y}{2}\Rightarrow\frac{2x}{6}=\frac{5y}{10}\)

Theo t/c dãy tỉ số = nhau:

\(\frac{2x}{6}=\frac{5y}{10}=\frac{2x-5y}{6-10}=\frac{-20}{-4}=5\)

=> \(\frac{x}{3}=5\Rightarrow x=5.3=15\)

=> \(\frac{y}{2}=5\Rightarrow y=5.2=10\)

Vậy x₀+y₀=15+10=25.

abc0 - abc = 1107

abc x 10 - abc = 1107

abc x (10 - 1) = 1107

abc x 9 = 1107

abc = 1107 : 9

abc = 123