we know that
If two triangles are congruent, then its corresponding angles and its corresponding sides are congruent
In this problem
triangle QRS and TRS are congruent -------> given
so
that means
by definition of congruence
by definition of congruence
by definition of congruence
QR=TR ---> by definition of congruence
RS=RS ---> by definition of congruence
QS=TS ---> by definition of congruence
Remember that
A triangle isosceles has two equal sides and two equal interior angles
In the triangle QST we have
QS=TS
and that means-----> the triangle has two equal sides and two equal interior angles
therefore
QST is an isosceles triangle
