What is the length of DE if DE || BC and D and E are midpoints of AB and AC?

 

Since In$∆ABC, D and E$, D and E are the given midpoints of the sides $AB and AC$ and$DE\parallel BC$, by using mid-point theorem,

$DE=\frac{1}{2}BC=\frac{1}{2}\times 18=9$

The length of DE is $9 cm$