# Transversal Trigonometry

### What Might This Be About?

9 September 2015, Created with GeoGebra

### Problem

The following has been posted by Oai Thanh Dào at the CutTheKnotMath facebook page:

### Proof

Draw lines through $A$ and $C$ parallel to $B_1C_1$ and then the perpendicular ones through $B$ an $C:$

In this particular case, we observe that

$AD=AB\cos (180^{\circ}-\gamma)=-AB\cos\gamma,\\ AG=AC\cos\beta,\\ DG=CE=BC\cos\alpha.$

Thus we see that, in this configuration, $DG=AG-AD,$ i.e.,

$BC\cos\alpha =AC\cos\beta + AB\cos\gamma.$

Other configurations are treated similarly.

### Application

Oai Thanh Dào has also supplied an application for the above:

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