直接两边对x求导就行了啊
5.e^(xy)(y+xy')+sec^2(xy)*(y+xy')=y'
(e^(xy)+sec^2(xy))xy'+(e^(xy)+sec^2(xy))y=y'
y'=(e^(xy)+sec^2(xy))y/(1-x(e^(xy)+sec^2(xy)))
6.y'=sec^2(x+y)*(1+y')
(1-sec^2(x+y))y'=sec^2(x+y)
y'=1/(cos^2(x+y)-1)=-1/sin^2(x+y)
y''=2/sin^3(x+y)*cos(x+y)*(1+y')
=2cos(x+y)/sin^3(x+y)*(1-1/sin^2(x+y))
=2cos(x+y)/sin^3(x+y)*(-cos^2(x+y))/sin^2(x+y)
=-2cos^3(x+y)/sin^5(x+y)
