The calculation results to problems 3-17 & 3-18 seem incorrect. I think the solution should be as follows:
Problem 3-17:
IN = - (IA + IB + IC)
= - (54.8 <(0 – 36.9⁰) + 0.25<(120⁰-5⁰) + 96<(240⁰-36.8⁰))
= -(-44.52 - j70.49)
= 44.52 + j70.49
= 83.37 <57.73⁰
Problem 3-18...
