Shortcut Trick
According to the Midpoint Theorem, the line segment connecting the midpoints of two sides of a triangle is parallel to the third side and is exactly half of its length.
Here, D and E are the midpoints of AB and AC respectively, and DE ∥ BC.
⇒ DE = BC ÷ 2
⇒ DE = 6 ÷ 2 = 3 cm
∴ The correct answer is 3 cm.
Alternate Method
Given:
In triangle ABC, D is the midpoint of AB and E is the midpoint of AC.
Length of base BC = 6 cm
DE ∥ BC
Formula Used:
By Basic Proportionality Theorem (Thales's Theorem) and similarity of triangles:
If DE ∥ BC, then ΔADE ∼ ΔABC.
The ratio of corresponding sides is: AD ÷ AB = AE ÷ AC = DE ÷ BC
Calculations:
Since D is the midpoint of AB:
⇒ AD = AB ÷ 2
⇒ AD ÷ AB = 1 ÷ 2
Since DE ∥ BC, in ΔADE and ΔABC:
⇒ ∠ADE = ∠ABC (Corresponding angles)
⇒ ∠AED = ∠ACB (Corresponding angles)
⇒ ΔADE ∼ ΔABC (By AA similarity criterion)
Therefore, the ratio of their corresponding sides is equal:
⇒ DE ÷ BC = AD ÷ AB
⇒ DE ÷ 6 = 1 ÷ 2
⇒ DE = 6 ÷ 2
⇒ DE = 3 cm
Let us analyze the other options to confirm why they are incorrect:
Option (2) 5 cm is incorrect because it does not satisfy the 1:2 ratio of the midpoint theorem.
Option (3) 2.5 cm is incorrect because 2.5 cm would mean BC is 5 cm, but BC is given as 6 cm.
Option (4) 6 cm is incorrect because DE must be strictly shorter than BC since D and E lie on the sides of the triangle.
∴ The correct answer is 3 cm.
Additional Information
Midpoint Theorem
The line segment connecting the midpoints of any two sides of a triangle is parallel to the third side and is half of its length.
Converse of Midpoint Theorem
The line drawn through the midpoint of one side of a triangle, parallel to another side, bisects the third side.
Basic Proportionality Theorem (Thales's Theorem)
If a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.