Proof of Sine Geometry and Angular Summing Cosinus

By: Al Azhary Masta, S.Si and Wulandari.

When children in high school learn trigonometry, sin()sincoscossinα+β=αβ+αβ and cos()coscossinsinα+β=αβ−αβ. However, the authors suspect that not many teachers prove to students why the equation is true? Sometimes students ask if that really applies? Where did he get it?

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