φ(t)=acost,ψ(t)=bsint,φ'(t)=-asint,ψ'(t)=bcost,φ"(t)=-acost,ψ"(t)=-bsint,φ'3(t)=asint
故d^2y/dx^2=（-abcost*cost-absint*sint)/asint=-b/sint