Question

Answer 1

do DE//BC => DE/BC = AD/AB <=> 3/7,5 = 2/(2+x)

=> 2+x = 5 => x = 3 

Answer 2

Theo định lý Talet: `(AD)/(AB) = (DE)/(BC)`.

`<=> 2/(2+x) = 3/(7,5)`

`<=> 15= 6 + 3x`

`<=> x = 3`.

Answer 3

\(\text{Xét }\Delta ABC\text{ có:}\)

\(DE//BC\left(gt\right)\)

\(\Rightarrow\dfrac{DE}{BC}=\dfrac{AD}{AB}\left(\text{định lí Ta-lét}\right)\)

\(\Rightarrow\dfrac{3}{7,5}=\dfrac{2}{2+x}\)

\(\Rightarrow2+x=\dfrac{7,5.2}{3}=5\)

\(\Rightarrow x=5-2=3\Rightarrow DB=3\left(cm\right)\)