第2个回答 2020-08-17
已知 y=y(x),x=sint;
dy/dt=(dy/dx)(dx/dt)=(dy/dx)cost=y'cost;其中,y'=dy/dx; 注意y'是x的函数。
d²y/dt²=d(dy/dt)/dt=d(y'cost)/dt=costdy'/dt+y'd(cost)/dt
=cost(dy'/dx)(dx/dt)-y'sint=cost(d²y/dx²)(dx/dt)-(dy/dx)sint
=cos²t(d²y/dx²)-(dy/dx)sint=(1-sin²t)(d²y/dx²)-(dy/dx)sint
=(1-x²)(d²y/dx²)-x(dy/dx);
其中,dy'/dx就是y对x的二阶导数,故dy'/dx=d(dy/dx)/dx=d²x/dx²;