#Python 2, 89 named / 87 unnamed

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 f=lambda n:"\n".join([" "+n/2*4*"_",(" /\\ "*n)[:2+2*n],("/__\\"*n)[:2*n-~(n%2)]][n<2:])

This is a function which takes in an int `n` and returns the triangles as a string using the row-by-row approach.

e.g. `print f(10)` gives

 ____________________
 /\ /\ /\ /\ /\ /
 /__\/__\/__\/__\/__\/