y'=[sin(x+y)]'(x+y)'=(1+y')cos(x+y)=cos(x+y)+y'cos(x+y)

　　y'=cos(x+y)/[(1-cos(x+y)]

　　y''=[cos(x+y]'(x+y)'+y''cos(x+y)+y'[cos(x+y)]'

　　=-(1+y')sin(x+y)+y''cos(x+y)-y'(1+y')sin(x+y)

　　y''[cos(x+y)-1]=(1+y')^2sin(x+y)

　　={1+cos(x+y)/[(1-cos(x+y)]}^2sin(x+y)

　　=sin(x+y)/[cos(x+y)-1]^2

　　y''=sin(x+y)/[cos(x+y)-1]^3