令-π/2+2kπ≤2x+φ≤π/2+2kπ,得：-π/4-φ/2+kπ≤x≤π/4-φ/2+kπ
那么有-π/4-φ/2+kπ=-5π/12+kπ,π/4-φ/2+kπ=π/12+kπ,解得：φ=π/3.