来自《耶鲁大学开放课程：基础物理 第16集 第16集》

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  or you can talk about the length of the vector and the angle it makes.
  也可以用向量的长度和幅角来定义
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  Then, it's clear that x=r·cos(θ), and y=r·sin(θ),
  明显地 x=r·cos(θ) 而 y=r·sin(θ)
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  where θ is now an angle associated with a complex number
  这里 θ 是跟某一复数有关的角度
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  which tells you at what angle it's located,
  告诉我们复数与横轴的夹角
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  and r is the length of the complex number.
  而 r 就是该复数的模长
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  It's called the polar form of the complex number.
  这也被称为复数的极坐标表示法
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  Then, you can see that z, which is x+i·y,
  而后就会发现 z 也就是 x+i·y
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  is equal to r·(cos(θ)+i·sin(θ))
  就等于 r·(cos(θ)+i·sin(θ))
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  which you can now write as r·e^(i·θ).
  也可以写做 r·e^(i·θ)
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  That's why we did all that work,
  这就是我们为何要讲这么多
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  to tell you that a complex number can be written in this form.
  就是为了用这种形式来表示复数
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  You can either write this x+i·y, or else r·e^(i·θ).
  你可以写为 x+i·y 或是 r·e^(i·θ)