y"=2yy'化为：y"=(y^2)'积分：y'=y^2+c1代入y'(0)=2,得：2=1+c1,得：c1=1故y'=y^2+1dy/(y^2+1)=dx积分：arctany=x+c2代入y(0)=1得 c2=π/4即有y=tan(x+c2)=tan(x+π/4)
