USe Lyapunov's direct method to analyze the stability of the origin for the system {x' = -3x^3 - 5xy,
{y' = x^2 - y^3.
and the zero solution of the equation x'' + x'cos(x) + sin(x).
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Show that {x'= -y + x^2,
{y' = x.
is a conservative system.