This paper discussed an elementary solution to the envelope of a kind of family of curves of which the equation was one of the cubic equations in regard to a parameter.
本文论述曲线族的方程为关于参数的三次方程时，其包络曲线方程的初等求法。
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Concrete stress-strain curves under cyclic loads include not only envelope curve, but also unloading and re-loading curves.