另一件关于二重积分的是，我们已经讲过了，如何做更复杂的变量变换。
OK, now another thing we've seen with double integrals is how to do more complicated changes of variables.
此外，本文提出的LQ定理使我们能用相关分析法，通过变量变换，把单因素非线性回归问题，化成线性形成来处理。
Besides, the LQ theorem presented in this paper can be used to change a nonlinear single regression problem to a linear one by means of transformation of variables.
所用的可靠度计算公式直接在基本变量空间中求解，避免了繁琐的求偏导数和正交变换工作。
The formula of reliability index is calculated in the origin space directly, so derivation and orthogonal transformation are avoided.
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