When I am reading the paper about Rayleigh instability, I found this type of expanding method.(adsbygoogle = window.adsbygoogle || []).push({});

[tex]

\sqrt{1+(\frac{2\pi\delta}{\lambda})^2 \cos^2(\frac{2\pi x}{\lambda})} = 1 + \frac{1}{2}(\frac{2\pi\delta}{\lambda})^2\cos^2 (\frac{2\pi x}{\lambda}) + \cdots

[/tex]

Can someone tell me what type of expansion is this?

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# What type of expansion is this?

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