Consider triangles STK and RTL:
TK=TL, ∠1=∠2, ∠3=∠4 ⇒ ΔSTK = ΔRTL (ASA definition)
In congruent triangles all corresponding sides and are congruent ⇒
ST = TR
SK = RL = x
Consider triangles RTK and STL:
SL = x + KL and RK = x + KL ⇒ SL = RK
Now we have:
SL = RK
ST = TR
TL = TK
So ΔSTL = ΔRTK (SSS definition).
