Shortcut Trick
Given DE || BC and AD : DB = 7 : 9.
In ΔABC, AB = AD + DB = 7 + 9 = 16.
Since ΔADE ∼ ΔABC, DE/BC = AD/AB = 7/16.
As DE || BC, ΔDEF is similar to ΔCBF (by AA similarity).
Ratio of areas of similar triangles = (Ratio of corresponding sides)2.
Area(ΔDEF) : Area(ΔCBF) = (DE/BC)2 = (7/16)2 = 49 : 256.
∴ The correct answer is 49 : 256.
Alternate Method
Given: In ΔABC, DE || BC, AD : DB = 7 : 9, and CD, BE intersect at F.
Formula Used: For two similar triangles, the ratio of their areas is equal to the square of the ratio of their corresponding sides.
Calculations:
⇒ Let AD = 7x and DB = 9x.
⇒ Total length AB = AD + DB = 7x + 9x = 16x.
⇒ In ΔADE and ΔABC, ∠A is common and ∠ADE = ∠ABC (corresponding angles as DE || BC).
⇒ ∴ ΔADE ∼ ΔABC (by AA similarity).
⇒ DE/BC = AD/AB = 7x / 16x = 7/16.
⇒ Now, in ΔDEF and ΔCBF:
⇒ ∠EDF = ∠BCF (Alternate interior angles).
⇒ ∠DEF = ∠CBF (Alternate interior angles).
⇒ ∴ ΔDEF ∼ ΔCBF (by AA similarity).
⇒ Area(ΔDEF) / Area(ΔCBF) = (DE / BC)2
⇒ Area Ratio = (7 / 16)2 = 49 / 256.
∴ The correct answer is 49 : 256.
Additional Information
Basic Proportionality Theorem (Thales Theorem)
If a line is drawn parallel to one side of a triangle intersecting the other two sides, it divides the two sides in the same ratio (AD/DB = AE/EC).
Area of Similar Triangles
The ratio of the areas of two similar triangles is equal to the square of the ratio of any two corresponding sides, altitudes, or medians.
AA Similarity Criterion
If two angles of one triangle are equal to two angles of another triangle, then the triangles are similar.