# Mathematics » High School: Number & Quantity » The Complex Number System

## Perform arithmetic operations with complex numbers.

• N-CN.1. Know there is a complex number i such that i2 = –1, and every complex number has the form a + bi with a and b real.
• N-CN.2. Use the relation i2 = –1 and the commutative, associative, and distributive properties to add, subtract, and multiply complex numbers.
• N-CN.3. (+) Find the conjugate of a complex number; use conjugates to find moduli and quotients of complex numbers.

## Represent complex numbers and their operations on the complex plane.

• N-CN.4. (+) Represent complex numbers on the complex plane in rectangular and polar form (including real and imaginary numbers), and explain why the rectangular and polar forms of a given complex number represent the same number.
• N-CN.5. (+) Represent addition, subtraction, multiplication, and conjugation of complex numbers geometrically on the complex plane; use properties of this representation for computation. For example, (-1 + √3 i)3 = 8 because (-1 + √3 i) has modulus 2 and argument 120°.
• N-CN.6. (+) Calculate the distance between numbers in the complex plane as the modulus of the difference, and the midpoint of a segment as the average of the numbers at its endpoints.

## Use complex numbers in polynomial identities and equations.

• N-CN.7. Solve quadratic equations with real coefficients that have complex solutions.
• N-CN.8. (+) Extend polynomial identities to the complex numbers. For example, rewrite x2 + 4 as (x + 2i)(x – 2i).
• N-CN.9. (+) Know the Fundamental Theorem of Algebra; show that it is true for quadratic polynomials.