Brachistochrone Curve trigonometric subsitution

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Hi I am currently trying to understand the Brachistrone Problem. I was looking at this video: https://youtu.be/zYOAUG8PxyM and at 7:53, there is a substitution for $y= h-C1*\sin^2(\theta/2)$. If someone could please explain how and why the creator substituted this and maybe where this trigonometric substitution comes from. Thank you so much:)