Need help with a triangle's length side/ angle relation.

[Dyatlov]

Hello.
I am trying to wrap my head around where from did he got the x = sin of theta equation at the 32:44 mark of the video: .
Isn't sine of theta x over the hypotenuse in the diagram ?
Thanks in advance!

 

[mfb]

Isn't sine of theta x over the hypotenuse in the diagram ?

Yes it is, and the length of the hypotenuse (=the pendulum arm) seems to be 1.