dx/dt=cost dy/dx=(dy/dt)/(dt/dx)=-3sin3t/cost=-3*(sinπ/4)/cosπ/4=-3 y=-cosπ/4=-2^(1/2),x=sinπ/4=2^(-1/2) 切线方程：y-[-2^(-1/2)]=-3[x-2^(-1/2)] y=-3x+2*2^(-1/2)=-3x+2^(1/2)
