(1)(2a-c)cosB=bcosC 正弦定理得:(4RsinA-2RsinC)cosB=2RsinBcosC 2sinAcosB=sinBcosC+sinCcosB 2sinAcosB=sin(B+C) 2sinAcosB=sinA 则：cosB=1/2 得B=π/3(2)由于：A+B+C=π,B=π/3则：A属于(0,2π/3)m*n=(cosA,cos...