求（b-2c）/[acos(60°+C)].
（1）f(x)=ab=2sinxcosx+2(cosx)^2
=sin2x+cos2x+1=v2sin(2x+π/4)+1
所以周期为π
（2）当2x+π/4=2kπ+π/2,即：x=kπ+π/8
f(x)|max=v2+1
当2x+π/4=2kπ-π/2,即：x=kπ-3π/8
f(x)|min==-v2+1;
(3) 当当2kπ-π/2=