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  • Qiwei Ye thinks this is interesting:

Here, we’ve created three triangles using only five vertices. The triangles shown in Figure 5.2 are defined using the following vertices:

From

Cover of Learning iOS Game Programming

Note

It seems that this example is wrong.
The right triangles is like below:

Triangle 1 = 1,2,3
Triangle 2 = 3,2,4
Triangle3 = 3,4,5