若(ta+b)∥c,则（2-2t）/2=(2t+1)/(-1)
t=-2
|a-tb|=3
(-2-2t)^2+(2-t)^2=9
t=-1或1/5
f(x)=a·b=(2m sinx,n cosx)*(cosx,2cosx)=2m sinx*cosx+2n(cosx)^2
=msin2x+ncos2x+n
f(π/6)=12
√3m+3n=24
f(π/3)=8
√3m+n=16
m=4√3 n=4
f(x)=4√3sin2x+4cos2x+4=8sin(2x+π/6)+4
最小周期T=π 增区间[-π/3+kπ,π/6+kπ] k∈R