b=(8sinx,2sinx+1)?
1
f(x)=a·b=(cosx,4sinx-2)·(8sinx,2sinx+1)=8sinxcosx+8sinx^2-2+4sinx-4sinx
=4sin(2x)+4(1-cos(2x))-2=4sqrt(2)sin(2x-π/4)+2,当：sin(2x-π/4)=1时
f(x)取得最大值：4sqrt(2)+2
2
f(A)=4sqrt(2)sin(2A-π/4)+2=6,即：sin(2A-π/4)=sqrt(2)/2,0