Question

\(\)Tìm x; y biết:

\(\frac{x}{3}=\frac{y}{7}\) và \(x.y=84\)

Answer 1

Ta có: xy = 84

=> \(y=\frac{84}{x}\)

=> \(\frac{x}{3}=\frac{\frac{84}{x}}{7}\)

=> \(\frac{x}{3}=\frac{12}{x}\)

=> \(x^2=3.12=36\)

=> \(x=\pm6\)

Khi x = 6 

=> \(y=\frac{84}{x}=\frac{84}{6}=14\)

Khi x = -6

=> \(y=\frac{84}{x}=\frac{84}{-6}=-14\)

Answer 2

Theo bài ra ta có: \(\frac{x}{3}=\frac{y}{7}\Rightarrow\frac{x}{3}.\frac{x}{3}=\frac{y}{7}.\frac{y}{7}=\frac{x}{3}.\frac{y}{7}\)

\(\Rightarrow\frac{x^2}{9}=\frac{y^2}{49}=\frac{84}{21}=4\)

\(\Rightarrow x^2=4.9=36\Rightarrow x=\pm6\)

\(\Rightarrow y^2=196=\pm14\)

Vậy \(x=\pm6\)

\(y=\pm14\)

 

Answer 3

\(\frac{x}{3}=\frac{y}{7}\)

\(\Rightarrow x=3;y=7\)

\(x.y=84\)

\(\Rightarrow x=\frac{84}{y}\)

Answer 4

ADTCDTSBN:

x/3=y/7=x*y/3*7=84/21=4

NEU X/3=4 THI X=4*3=12

NEU Y/7=4 THI Y=7*4=28

VAY X=12;Y=28

Answer 5

Đặt \(\frac{x}{3}=\frac{y}{7}=k\left(k\ne0\right)\)

=> x =  3k;    y =7k

=> x.y = 3k.7k = \(21k^2=84\) 

=>\(k^2=4\)

=.\(\orbr{\begin{cases}k=2\\k=-2\end{cases}}\)

=>\(\orbr{\begin{cases}x=2.3=6\\x=-2.3=-6\end{cases}}\)

=>\(\orbr{\begin{cases}y=2.7=14\\y=-2.7=-14\end{cases}}\)

Answer 6

\(\frac{x}{3}=\frac{y}{7}\Leftrightarrow x=3k;y=7k;\Rightarrow xy=21k^2\Leftrightarrow k^2=4\Leftrightarrow k\in\left\{-2;2\right\}\Leftrightarrow\)

\(x,y\in\left\{\left(-6;-14\right);\left(6;14\right)\right\}\)

Answer 7

Gọi \(\frac{x}{3}=\frac{y}{7}=k\)

=>\(\hept{\begin{cases}x=3k\\y=7k\end{cases}}\)

Ta có:

\(x.y=3k.7k=21k^2=84\)

=>\(k=2\)

=>\(x=3.2=6\)

    \(y=7.2=14\)

Vậy x=6 và y=14