如题,明早前就要,请速回答,

21。f(x)=m*n=√3cos(x/4)sin(x/4)+(cos(x/4))^2=(√3/2)sin(x/2)+(1/2)*cos(x/2)+1/2
=sin((x/2)+30°)+1/2
因为有(2a-c)cosB=b*cosC,利用正弦定理进行边化角,得
2sinA*cosB-sinC*cosB=sinB*cosC
2sinA*cosB=sin(B+C)=sinA
cosB=1/2 B=60°
A的取值范围是0