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Step-by-step explanation:
The RHS congruence theorem states that, if the hypotenuse and side of one right triangle are equal to the hypotenuse and the corresponding side of another right triangle, the two triangles are congruent.
This is also known as the HL (hypotenuse-leg) congruence theorem for right triangles.
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The four theorems generally available for showing congruence of triangles are abbreviated ...
SSS, ASA, SAS, AAS . . . . A=angle, S=side; order is important
These list the sides/angles that must be identified as corresponding congruent parts.
Notable for its absence from this list is an SSA theorem. The reason is that SSA congruence can only be shown under the specific circumstance that the angle is opposite the longest of the two sides. That can only be guaranteed if the angle is a right angle or obtuse angle.
If the triangle is a right triangle, and the hypotenuse (longest side) is one of the two corresponding sides involved in the congruence statement, then the conditions required for RHS, or SSA, or HL congruence are established.
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Additional comment
In general, a triangle can be "solved" (all sides and angles determined) if at least one side and any two of the remaining angles or sides are specified. That is, the triangle will be uniquely specified.
The exception is described above, where two sides and an angle are given, but the side opposite the angle is the shorter of the two. In that case, if the triangle is not a right triangle, there are always two possible solutions. This is why, in general, congruence cannot be shown between two triangles with the same SSA specification.