The Beautiful Equation: Euler's identity.

上图：美丽的欧拉等式。

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Equation is different from arithmetic identity in that it contains unknown quantity which can join operations.

方程与算数的等式不同在于它含有可以参加运算的未知量。

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A power-like equation, a Virial-like theorem, an integral variational principle and a differential variational principle for the system are deduced by using the time-integral identity.

给出系统的时间积分等式，并由此等式导出类能量方程、类维里定理、一个积分变分原理和一个微分变分原理。

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Again, this equation is known as an identity equation: it is true for all countries and times.

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