ASA congruence theorem: If two angles and the included side of one triangle are equal to the corresponding angles and side of another triangle, the triangles are congruent.
AAS congruence theorem: If two angles and the non-included side of one triangle are equal to the corresponding angles and side of another triangle, the triangles are congruent.
Consider two triangles ΔABC and ΔA'B'C' . If AB=A'B', m∠A=m∠A' and m∠B=m∠B', then two triangles ΔABC and ΔA'B'C' are congruent by ASA theorem. Now find ∠C and ∠C': and . You have AB=A'B', m∠A=m∠A' , m∠C=m∠C' and that is the condition of AAS congruence theorem.
<span>These thoughts show you that AAS theorem is straight extension of ASA theorem.</span>
The interior angle measures of a triangle add up to 180 degrees. Thus, if you are given angle-angle-side, you can solve for the third angle measure and essentially have angle-side-angle because the given side will now be the included side.