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Continued from other part ...

If you looked at the document, imagine the bit above the black line wasn't there. Then, the whole perimeter would just be 2(a+b), believe it or not. But the bit above the black line is there, so all you do is add on 2c (because one of the side lengths is c and there are two of them). The length at the top doesn't count because it had already been counted as part of the perimeter. So there you have it. The perimeter would be 2(a+b+c).

Thanks for reading.