过A做AM垂直PC于M
易知
PA⊥BC,AC⊥BC
=>
BC⊥PAC
=>
BC⊥AM,BC⊥PC
又
AM⊥PC
=>
AM⊥PBC
cosA-PB-C = S(PMB)/S(PAB) = 1/2*S(PBC)/S(PAB) = 1/2*1/ √3/2
= 1/√3
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