过A作AE//PC交CD于E.
则由于AE//PC
所以∠APC+∠PAE=180°
∠PCD=∠DEA
又由于∠BAE+∠DEA=180°
所以∠APC+∠PAE+∠PCD+∠BAE=360°
而∠PAB+∠PAE+∠BAE=360°
所以∠PAB=∠APC+∠PCD.
过A作AE//PC交CD于E.
则由于AE//PC
所以∠APC+∠PAE=180°
∠PCD=∠DEA
又由于∠BAE+∠DEA=180°
所以∠APC+∠PAE+∠PCD+∠BAE=360°
而∠PAB+∠PAE+∠BAE=360°
所以∠PAB=∠APC+∠PCD.