Meowth 寫到:PA^2+PB^2=AB^2+2PA*PB*COS60'=a^2+PA*PB.....(1)
PA^2+PC^2=AC^2+2PA*PC*COS60'=a^2+PA*PC....(2)
PB^2+PC^2=BC^2+2PB*PC*COS120'=a^2-PB*PC...(3)
-(1)-(2)+(3)*2:
PB^2+PC^2-2PA^2=-2PB*PC-PA*(PB+PC)
(PB+PC)^2+PA*(PB+PC)-2PA^2=0
(PB+PC-PA)(PB+PC+2PA)=0
==> PB+PC=PA