Shortcut Trick
For two triangles to be similar, their corresponding angles must be equal according to the AA (Angle-Angle) Similarity Criterion.
Given: ∠A = ∠D = 40°.
To satisfy similarity, we need at least one more pair of corresponding angles to be equal.
From the options, ∠C = ∠F provides the necessary second pair of equal corresponding angles.
Since ∠A = ∠D and ∠C = ∠F, the triangles ΔABC and ΔDEF are similar by AA similarity.
∴ The correct answer is ∠C = ∠F.
Alternate Method
Given: In ΔABC and ΔDEF, ∠A = ∠D = 40° and AB = DE.
Formula Used: AA Similarity Criterion — If two angles of one triangle are equal to two angles of another triangle, then the triangles are similar.
To prove ΔABC ~ ΔDEF:
⇒ We already have one pair of equal angles: ∠A = ∠D = 40°.
⇒ According to the AA Similarity Criterion, we need another pair of corresponding angles to be equal.
⇒ The corresponding vertices are A → D, B → E, and C → F.
⇒ Therefore, either ∠B = ∠E or ∠C = ∠F must be true.
⇒ Checking the options, ∠C = ∠F is provided in option (1).
⇒ Note: Since AB = DE is also given, if ∠C = ∠F, then ∠B = ∠E (angle sum property), making the triangles congruent by ASA. Since all congruent triangles are similar, the condition is satisfied.
∴ The correct answer is ∠C = ∠F.
Additional Information
AA Similarity Criterion
If two angles of one triangle are respectively equal to two angles of another triangle, then the two triangles are similar.
SAS Similarity Criterion
If one angle of a triangle is equal to one angle of another triangle and the sides including these angles are proportional, then the triangles are similar.
Similarity vs Congruence
Congruent triangles have the same shape and size (ratio of sides = 1), while similar triangles have the same shape but may differ in size (ratio of sides is constant).