（1）、|a|=√[（sinx）^2+(cosx)^2]=1,|c|=1,
a•c=-cosx,
设向量a、c的夹角为α,
cosα= a•c/(|a|*|c|)=-cosx/1,
x=π/3,cosα=-cos(π/3)=-1/2,
α=120°,
（2）,a•b=-(cosx)^2+sinxcosx
=-(1+cos2x)/2+sin2x/2
=(sin2x-cos2x)-1/2
=√2[(√2/2)sin(2x)-√2/2cos(2x)]-1/2
=√2sin(2x-π/4)-1/2,
f(x)=2√2sin(2x-π/4)-1+1,
f(x)= 2√2sin(2x-π/4),
故f(x)∈[-2√2,2√2].