\(\frac{a-b}{b-c}=\frac{c-d}{d-a}=\left(a-b\right)\cdot\left(d-a\right)=\left(b-c\right)\cdot\left(c-d\right)=\)
\(\left(a-b\right)\cdot d-\left(a-b\right)\cdot a=\left(b-c\right)\cdot c-\left(b-c\right)\cdot d=\)
\(ad-bd-2a-ab=bc-2c-bd-cd\)
Suy ra a=c
Nhớ cho mình nha!!!