It seems to be the geometric interpretation of the law of cosine.
1)Let be the vector seen below.
2)Let be the other vector.
3)Then is the vector sum.
4)And is the reflection of the vector through the x-axis.
5)Then is the sum of those two vectors.
Let be the length of and let be the length of and let be the length of the line joining the tips of and .
Now and and .
If we can show that then, .
This seems to be a Needham type of problem.