两边对x求导:1-y'+0.5y'cosy=0,得:y'=1/(1-0.5cosy)
于对x求导:y"=-1/(1-cosy)^2*(1-0.5cosy)'
=-1/(1-cosy)^2*(0.5y'siny),再代入y'
=-0.5siny/(1-cosy)^3
两边对x求导:1-y'+0.5y'cosy=0,得:y'=1/(1-0.5cosy)
于对x求导:y"=-1/(1-cosy)^2*(1-0.5cosy)'
=-1/(1-cosy)^2*(0.5y'siny),再代入y'
=-0.5siny/(1-cosy)^3