The annual variation appears a tri-peak form in the curve of dekad- or pentad-hail days evolution.
从旬或候雹日演变看，冰雹年变化具有三峰型；
A group of equations of assembly datum variation is proposed to model assembly tolerance chain and tri-dimensional tolerance analysis result is obtained through solving those equations.
提出用装配基准变动方程组来建立装配公差链模型，通过求解此方程组获得三维装配公差链的分析结果。
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