Cho $\dfrac{a}{b} = \dfrac{b}{c} = \dfrac{c}{d}.$ Chọn đáp án đúng.

$\dfrac{x}{a} = \dfrac{y}{b} = \dfrac{{x + y}}{{a + b}}$

$\dfrac{x}{a} = \dfrac{y}{b} = \dfrac{{x.y}}{{a.b}}$

$\dfrac{x}{a} = \dfrac{y}{b} = \dfrac{{x.y}}{{a + b}}$

$\dfrac{x}{a} = \dfrac{y}{b} = \dfrac{{x - y}}{{a + b}}$

$\dfrac{x}{a} = \dfrac{y}{b} = \dfrac{z}{c} = \dfrac{{x + y + z}}{{a + b + c}}$

$\dfrac{x}{a} = \dfrac{y}{b} = \dfrac{z}{c} = \dfrac{{x - y - z}}{{a - b - c}}$

$\dfrac{x}{a} = \dfrac{y}{b} = \dfrac{z}{c} = \dfrac{{x - y + z}}{{a - b + c}}$

$\dfrac{x}{a} = \dfrac{y}{b} = \dfrac{z}{c} = \dfrac{{x + y - z}}{{a - b + c}}$

$x =  - 20;y =  - 12$

$x =  - 12;y = 20$      

$x =  - 12;y =  - 20$  

$x = 12;y =  - 20$

$27;\,33$

$33;27$

$27;44$

$27;34$

$y = 4;x = 7$

$x = 32;y = 56$

$x = 56;y = 32$

$x = 4;y = 7$

$6;\,12;14;\,18$

$18;14;10;6$ 

$6;14;10;18$

$6;10;14;18$

$27$

$- 27$

$- 18$

$- 45$