Answer:
DE = 6 cm
Step-by-step explanation:
Let DE = x cm.
Since DE is parallel to AB therefore by the alternate interior angles theorem, m∠BAD = m∠ADE and m∠ABE = m∠DEB ............(1)
As AD is an angle bisector of ∠A, therefore m∠EAD = m∠DAB ; Since BE is an angle bisector of ∠B ⇒ m∠ABE = m∠EBD.
Therefore, from (1) We get , m∠EAD = m∠ADE and m∠EBD = m∠BED.
So, the triangles ADE and EDB are then isosceles with AE = ED and ED = DB.
So AE = DE = DB = x, and since the perimeter of ABDE is 30 cm, then
12 + x + x + x = 30
⇒ 12 + 3x = 30
⇒ x = 6
Hence, the length of DE is 6 cm.
Answer:
DE=6cm
Step-by-step explanation:
Let x=DE, since AB║DE, therefore ∠BAD=∠ADE and ∠ABE=∠BED. (1)
Also, we are given that AD is the angle bisector of ∠A and BE is the angle bisector of ∠B,
Therefore, ∠EAD=∠DAB and ∠ABE=∠EBD (2)
From (1) and (2), ∠EAD=∠ADE and ∠EBD=∠BED
⇒The ΔADE and ΔEDB then becomes the isosceles triangle with AE=Ed and ED=DB (Sides opposite to equal angles are always equal)
Therefore, AE=DE=DB=x
We are given that the perimeter of ABDE is 30 cm, therefore,
Perimeter of ABDE= sum of all the sides of ABDE
⇒30=AB+BD+DE+AE
⇒30=12+3x
⇒30-12=3x
⇒x=6cm
Therefore, the length of DE= 6cm
