f(x)=sin2x+2cos(π-x)cosx+1
=sin2x-2cos^2x+1
=sin2x-cos2x
=√2sin(2x-π/4)
最小正周期T=π ,最大值y=√2
b^2+c^2=a^2+√2bc,a^2=b^2+c^2-2bccosA,a^2+√2bc=a^2-2bccosA,cosA=-√2/2,A=3π/4
f(3π/4)=√2sin(3π/2-π/4)=√2sin5π/4=-1