y=f(x)=3sin(2x+φ),令t=2x+φ,则f(t)=3sint,其对称点为(π+kπ,0)
又x=(t-φ)/2,令x=(t-φ)/2=5π/4,即t=φ+5π/2
令φ+5π/2=π+kπ,得φ=-3π/2+kπ,易得当k=1或2时|φ|=|φ|min=π/2