本文首先使用初等方法导出三角形与四边形的等周不等式，进而用富里埃级数方法解决了一般等周问题。
We deduced equal perimeter inequality of triangle and quadrangle by elementary mathematical methods first and solved general equal perimeter problems by using methods of Fourier series.
该方法不用一般的变分原理，可适用任意变系数正定和非正定偏微分方程。利用这一方法得到一个新的八节点四边形平面应力单元。
It doesn't need the variational principle and can be applied to solve non-positive or positive definite partial differential equations with arbitrary variable coefficient.
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