In circuit analysis, the relationship between sine and cosine functions, specifically the notion that i*cos = sin, is a misunderstanding. The correct relationship is that phasors represent these functions, where phasor notation shows that the phasor of cos(ωt) is j times the phasor of sin(ωt), but this does not imply that cos(ωt) equals j*sin(ωt). The discussions emphasize that while linear problems allow for certain substitutions, these do not hold in all contexts, particularly when dealing with time-domain functions versus phasor representations. Ultimately, using phasors and complex numbers is essential for accurate circuit analysis, as these methods provide a consistent and rigorous framework for understanding relationships between voltage and current. The conversation highlights the importance of adhering to established mathematical principles rather than relying on informal substitutions.