y'=sec^2(x+y)*(1+y')y'(1-sec^2(x+y))=sec^2(x+y)y'*tan(x+y)=-sec^2(x+y)y'=-1/cos^2(x+y)*1/tan(x+y)=-1/[sin(x+y)cos(x+y)]=-2/sin(2x+2y)=-2csc(2x+2y)y''=2cot(2x+2y)csc(2x+2y)*(2+2y')把y'代入即可