(1)如图,过点P向左作PQ∥AC,则∠APQ=∠PAC,
∵AC∥BD,
∴PQ∥BD,
∴∠BPQ=∠PBD,
∵∠APB=∠APQ+∠BPQ,
∴∠APB=∠PAC+∠PBD;
(2)不成立.∠APB+∠PAC+∠PBD=360°.
理由如下:如图,过点P向右作PQ∥AC,则∠APQ+∠PAC=180°,
∵AC∥BD,
∴PQ∥BD,
∴∠BPQ+∠PBD=180°,
∴∠APQ+∠PAC+∠BPQ+∠PBD=180°×2=360°,
∵∠APB=∠APQ+∠BPQ,
∴∠APB+∠PAC+∠PBD=360°