The expression "a + bi" represents a complex number, where "a" is the real part and "bi" is the imaginary part. In this format, "b" is a real number multiplied by the imaginary unit "i," which is defined as the square root of -1. Complex numbers are used in various fields, including engineering and physics, to solve equations that cannot be addressed with real numbers alone. Complex numbers can be added, subtracted, multiplied, and divided, following specific rules. For example, when adding two complex numbers, you combine their real parts and their imaginary parts separately. This mathematical framework allows for a deeper understanding of phenomena in electrical engineering and quantum physics, where traditional real-number calculations may fall short.