A complex number is a number that can be expressed in the form a + bi, where a and b are real numbers and i is the imaginary unit. The real part, a, represents the real number and the imaginary part, b, represents the imaginary number. Complex numbers can be added, subtracted, multiplied, and divided like real numbers, and they can also be raised to powers and have roots taken.Complex numbers have many applications in various branches of mathematics and physics. They are used in electrical engineering, signal processing, quantum mechanics, and computer science, among other fields. They can represent quantities such as voltages and currents in electrical circuits, the position and velocity of particles in quantum mechanics, and the coefficients of polynomials in computer science algorithms.Complex numbers are also used to describe geometric transformations, such as rotations and dilations, and can be represented by points in the complex plane. The complex plane is a two-dimensional coordinate system where the real axis represents the real part of a complex number and the imaginary axis represents the imaginary part. The magnitude or absolute value of a complex number can be found using the Pythagorean theorem as |z| = √(a^2 + b^2), and the argument or angle of a complex number can be found using trigonometry as θ = atan(b/a).Overall, complex numbers provide a powerful tool for solving problems in both mathematics and science, allowing for the representation and manipulation of quantities that cannot be expressed solely by real numbers.