Two triangles are similar if their  sides are proportional.

If $D$ and $E$ are midpoints of the sides $AB$ and $AC$ of the $∆ABC$. Then $AD:AB=$_____.

Two equilateral triangle are always _____ .

Two isosceles triangles are always similar.

In a triangle $ABC$ if $D$ and $E$ are two points on $AB$ and $AC$ such that $DE\parallel BC$, then $AD, BD, AE$ and $CE$ are in proportion.