Introduction The product of two complex numbers is found using the same method for multiplying two binomials. As when multiplying binomials, both terms in the first complex number need to be multiplied by both terms in the second complex number. The product of the two binomials x + y and x – y is the difference of squares: x 2 – y 2. If y is an imaginary number, this difference of squares will be a real number since i i = –1: (a + bi)(a – bi) = a 2 – (bi) 2 = a 2 – b 2 (–1) = a 2 + b : Multiplying Complex Numbers

Key Concepts Simplify any powers of i before evaluating products of complex numbers. In the following equations, let a, b, c, and d be real numbers : Multiplying Complex Numbers

Key Concepts, continued Find the product of the first terms, outside terms, inside terms, and last terms. Note: The imaginary unit i follows the product of real numbers. (a + bi ) (c + di ) = ac (product of the first terms) + adi (product of the outside terms) + bci (product of the inside terms) + bidi (product of the last terms) = ac + adi + bci + bdi 2 = ac + bd(–1) + adi + bci = (ac – bd) + (ad + bc)i : Multiplying Complex Numbers

Key Concepts, continued ac – bd is the real part of the product, and ad + bc is the multiple of the imaginary unit i in the imaginary part of the product. A complex conjugate is a complex number that when multiplied by another complex number produces a value that is wholly real. The product of a complex number and its conjugate is a real number : Multiplying Complex Numbers

Key Concepts, continued The complex conjugate of a + bi is a – bi, and the complex conjugate of a – bi is a + bi. The product of a complex number and its conjugate is the difference of squares, a 2 – (bi) 2, which can be simplified. a 2 – b 2 i 2 = a 2 – b 2 (–1) = a 2 + b : Multiplying Complex Numbers

Common Errors/Misconceptions incorrectly finding the product of two complex numbers incorrectly identifying the complex conjugate of a + bi as a value such as –a + bi, a + bi, or –a – bi : Multiplying Complex Numbers

Guided Practice Example 1 Find the result of i 2 5i : Multiplying Complex Numbers

Guided Practice: Example 1, continued 1.Simplify any powers of i : Multiplying Complex Numbers

Guided Practice: Example 1, continued 2.Multiply the two terms. Simplify the expression, if possible, by simplifying any remaining powers of i or combining like terms. 5(–i) = –5i : Multiplying Complex Numbers ✔

Guided Practice: Example 1, continued : Multiplying Complex Numbers

Guided Practice Example 2 Find the result of (7 + 2i)(4 + 3i) : Multiplying Complex Numbers

Guided Practice: Example 2, continued 1.Multiply both terms in the first polynomial by both terms in the second polynomial. Find the product of the first terms, outside terms, inside terms, and last terms : Multiplying Complex Numbers

Guided Practice: Example 2, continued 2.Evaluate or simplify each expression : Multiplying Complex Numbers

Guided Practice: Example 2, continued 3.Combine any real parts and any imaginary parts : Multiplying Complex Numbers ✔

Guided Practice: Example 2, continued : Multiplying Complex Numbers