x=e^(-t), dx/dt = -e^(-t) = -x
dy/dx = (dy/dt) / (dx/dt) = (-1/x) * dy/dt
d²y/dx² = (1/x²) * dy/dt + (-1/x) * d²y/dt² / (dx/dt) = (1/x²) * dy/dt + (1/x²) * d²y/dt²
= (1/x²) * [ dy/dt + d²y/dt² ]
x * dy/dx = - dy/dt, x² * d²y/dx² = dy/dt + d²y/dt²
代入原方程,得:d²y/dt² + y= 0
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