(2a-c)cosB=bcosC
(2sinA-sinC)cosB=sinBcosC
2sinAcosB-(sinCcosB+sinBcosC)=0
2sinAcosB-sin(C+B)=0
2sin(C+B)cosB-sin(C+B)=0
sin(C+B)(2cosB-1)=0
sin(C+B)不等于0
2cosB-1=0
cosB=1/2
B=60
m•n=4KsinA+cos2A=-2sin^2A+4KsinA+1
=-2(sinA-K)^2+2K^2+1
m•n的最大值为5,
2K^2+1=5,k>1
K=根号2