f(x)=Asin(wx+ψ)由周期的定义
f(x+t)=f(x)
即是：Asin(w（x+t)+ψ)=Asin(wx+ψ)
Asin(wx+ψ+wt)=Asin(wx+ψ)
根据正弦函数的性质 wt=2kπ
t=2kπ/w
k=1时,最小正周期T=2π/w