angle(PAB) is a right angle because it is formed by a diameter and a tangent
angle(ACB) is a right angle because it intercepts a semicircle.
angle(PCA) is a right anlge because it forms a straight line with angle(ACB).
Therefore, angle(PAB) is congruent to angle(PCA).
angle(P) is congruent to angle(P) by identity.
Therefore, the two triangles are similar by AA.