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## Homework Statement

Hi everybody! In the middle of an exercise, our teacher suddenly wrote:

[tex]

sin(\frac{x}{y} sin y) = \frac{x}{y} sin y - \frac{1}{2} sin θ (\frac{x}{y} sin y)^2

[/tex]

I don't get where does that come from? The closest I've managed to reach is:

[tex]

sin(\frac{x}{y} sin y) = \frac{x}{y} sin y - (\frac{x}{y} sin y)^2 \frac{x sin y}{6y} + O(x^5)

[/tex]

What is this θ? It doesn't show up neither before nor after that line :/

Thanks in advance for your answers.

Julien.