(1)∵m⊥n ∴(2a-c)cosB-bcosC=0(2sinA-sinC)cosB-sinBcosC=02sinAcosB=sin(C+B)2sinAcosB=sinAcosB=1/2∴B=60°(2)cosB=(a^2+c^2-b^2)÷(2ac)=1/2 c=5,c=-2(舍去)S=1/2acsinaB=15根号3/4
B=60°15跟号3/4