研究了用方向加速法来进行结构可靠指标的计算方法，该法可以避免对功能函数进行导数计算，并已算例说明该方法的正确性。

Powell method is used in the paper to calculate the reliability index, this method can avoid the complexity of partial derivative calculating, and the example shows that this method is correct.

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既然在y方向上,没有加速度，法向力也等于mgcosθ

Since there is no acceleration in the y direction, the normal force must be also mg cosine theta.

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