Denote ∠CAB=α, ∠ABC=β, and ∠BCA=γ. By tangency, ∠KAH=∠ABH=90∘−α (Fig. 41). Since ∠ACH=90∘−α as well, triangles AHC and KHA are similar. Consequently, the corresponding third angles are equal, i.e., ∠AKH=∠CAH=90∘−γ. Similarly, we get ∠LAH=90∘−α and ∠ALH=90∘−β. Thus, ∠KAL=∠KAH+∠LAH=180∘−2α, with AH bisecting the angle KAL.
On the other hand,
∠KHL=∠CHB=180∘−∠BCH−∠CBH=180∘−(90∘−γ)−(90∘−β)=β+γ=180∘−α.
Now note (Fig. 42) that for the incenter I of triangle AKL,
∠KIL=180∘−∠IKL−∠ILK=180∘−2∠AKL−2∠ALK=180∘−2∠AKL+∠ALK=180∘−2180∘−∠KAL=180∘−2180∘−(180∘−2α)=180∘−α,
so ∠KIL=∠KHL. Since points H and I lie on the same side of line KL, points K, H, I, and L lie on the same circle. Given that both H and I lie on the angle bisector AH of angle KAL, which intersects the chord KL, it follows that H=I. Consequently, ∠LKH=∠AKH=90∘−γ and ∠KLH=∠ALH=90∘−β. In addition, we have ∠HLK=90∘−β=∠HCB, from which it follows that points B, C, K, L lie on the same circle.