Shortcut Trick
The standard congruence criteria for triangles are SSS, SAS, ASA, AAS, and RHS.
The condition SSA (Two sides and a non-included angle) is not a valid criterion for congruence.
In the SSA case, it is possible to construct two different triangles using the same measurements, known as the ambiguous case.
Unless the angle is specifically the included angle (forming the SAS rule), the triangles are not necessarily congruent.
∴ The correct answer is If two sides and an angle of one triangle are equal to two sides and an angle of the other triangle, then the two triangles are congruent.
Alternate Method
Given: Various criteria for triangle congruence.
Properties Used: SAS (Side-Angle-Side) axiom and the Ambiguous Case of triangles.
⇒ For congruence, the SAS rule state that the angle must be the included angle between the two equal sides.
⇒ If the angle is not included (SSA), we can have a situation as shown in the diagram.
⇒ Consider ΔABC and ΔABD. They both share side AB (c), side BC = BD (a), and ∠A (35°).
⇒ Even though they have two sides and one angle equal, ΔABC is clearly not congruent to ΔABD.
⇒ This proves that two sides and an angle (non-included) do not guarantee congruence.
∴ The correct answer is If two sides and an angle of one triangle are equal to two sides and an angle of the other triangle, then the two triangles are congruent.
Additional Information
SAS Congruence Criterion
Two triangles are congruent if two sides and the included angle of one triangle are equal to the corresponding sides and the included angle of the other triangle.
ASA and AAS Rules
Two triangles are congruent if two angles and any side (included or non-included) are equal, as the third angle is automatically equal by the angle sum property.
RHS Congruence Criterion
For right-angled triangles, if the hypotenuse and one side of one triangle are equal to the hypotenuse and one side of another, they are congruent (this is a special case of SSA).
The SSA Ambiguous Case
SSA does not guarantee congruence because, depending on the side lengths, zero, one, or two distinct triangles can be formed with the same given parts.