Answer:
DE = 13.4 cm (to 1 decimal place)
Explanation:
Given: ABCD is a square
BC = AC = 12 cm (opposite sides of a square are congruent)
E is midpoint of BC (given)
BE = EC = 12/2 = 6 cm
CD = AB = 12 cm (opposite sides of a square are congruent)
angle ECD is a right angle (interior angles of a square are 90 deg.)
Consider right triangle ECD
DE = sqrt(EC^2+CD^2) ............. pythagorean theorem
= sqrt(6^2+12^2)
= sqrt ( 36+144 )
= sqrt (180)
= 2 sqrt(45)
= 13.416 (to three dec. places)