Triangle having its vertices on three concurrent lines r1, r2, r3 and as incenter I their concurrence point
1) Take A in r1.
2) Let A' be the reflection of A
on r2.
3) Let A" be the reflection of A'
on r3.
4) The line AA" cuts r3 at C and
the line A'C cuts r2 at B; ABC is the requested triangle.
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