proof of line current and phase current * 1.73 signs problem

Hi, I need some help

trying to proof that the I line = 1.73 * I phase
Can't understand the signs of the 10 AMP in the bottom left one
like why we used +10 in the A phase and -10<240 from A to B, I think.
this is the whiteboard for a clear overview :
https://wbd.ms/share/v2/aHR0cHM6Ly93aGl0ZWJvYXJkLm1pY3Jvc29mdC5jb20vYXBpL3YxLjAvd2hpdGVib2FyZHMvcmVkZWVtLzlhZmUzMDM5YmRkNjQ4ODZhNGE0ZmI4NGIyZDk2YjFiX0JCQTcxNzYyLTEyRTAtNDJFMS1CMzI0LTVCMTMxRjQyNEUzRF8wNTRiNWE1NS1hM2IzLTRkNTYtOTc3My1jZGU1NWE4Yzc3YmU=