If D and E are respectively the midpoints of the sides AB and BC of ∆ABC in which AB = 7.2 cm, BC = 9.8 cm and AC = 3.6

In ∆">Δ∆ABC,

Since, D and E are respectively the mid-points of sides AB and BC.     (Given)

So, $D E=\frac{1}{2} A C$       (Uing mid-point theorem)

But AC = 3.6 cm      (Given)

$D E=\frac{1}{2}(3.6)$

or, DE = 1.8 cm

Hence, the length of DE is 1.8 cm.