sinA(wt+Ψ)Ψ就是初相角
A=2T/4=(5π/12)-(3π/12)=π/6T=2π/3=2π/ωω=3y=2sin(3x+φ)再把(π/4,0)代入到函数中得：0=2sin(3π/4+φ)φ=π/4y=2sin(3x+π/4)初相就是x=0时，y的值，y=2sin(π/4)=√2
