Introduction Finding sums and differences of complex numbers

Introduction Finding sums and differences of complex numbers is similar to finding sums and differences of expressions involving numeric and algebraic quantities. The real parts of the complex number are similar to the numeric quantities, and the imaginary parts of the complex number are similar to the algebraic quantities. Before finding sums or differences, each complex number should be in the form a + bi. If i is raised to a power n, use the remainder of n ÷ 4 to simplify in. 1 4. 3. 2: Adding and Subtracting Complex Numbers

Key Concepts • First, find the sum or difference of the real parts of the complex number. • Then, to find the sum or difference of the imaginary numbers, add or subtract the coefficients of i. • The resulting sum of the real parts and the imaginary parts is the solution. • In the following equation, let a, b, c, and d be real numbers. (a + bi) + (c + di) = a + c + bi + di = (a + c) + (b + d)i 2 4. 3. 2: Adding and Subtracting Complex Numbers

Key Concepts, continued • a + c is the real part of the sum, and (b + d)i is the imaginary part of the sum. • When finding the difference, distribute the negative throughout both parts of the second complex number. (a + bi) – (c + di) = a + bi – c – di = (a – c) + (b – d)i • a – c is the real part of the difference, and (b – d)i is the imaginary part of the difference. • The sum or difference of two complex numbers can be wholly real (having only real parts), wholly imaginary (having only imaginary parts), or complex (having both real and imaginary parts). 4. 3. 2: Adding and Subtracting Complex Numbers 3

Common Errors/Misconceptions • failing to distribute a negative throughout both the real and imaginary parts of a complex number before simplifying a difference • adding the multiples of two powers of i when the powers of i are not equal, such as 2 i 2 + 3 i = 5 i 4 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice Example 2 Is (5 + 6 i 9) – (5 + 3 i 15) wholly real or wholly imaginary, or does it have both a real and an imaginary part? 5 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 2, continued 1. Simplify any expressions containing in. Two expressions, 6 i 9 and 3 i 15, contain in. Divide each power of i by 4 and use the remainder to simplify in. 9 ÷ 4 = 2 remainder 1, so 9 = 2 • 4 + 1. i 9 = i 2 • 4 • i 1 = i 15 ÷ 4 = 3 remainder 3, so 15 = 3 • 4 + 3. i 15 = i 3 • 4 • i 3 = –i 6 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 2, continued Replace each occurrence of in in the expressions with the simplified versions, and replace the original expressions in the difference with the simplified expressions. 6 i 9 = 6 • (i) = 6 i 3 i 15 = 3 • (–i) = – 3 i (5 + 6 i 9) – (5 + 3 i 15) = (5 + 6 i) – [5 + (– 3 i)] 7 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 2, continued 2. Distribute the difference through both parts of the complex number. 8 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 2, continued 3. Find the sum or difference of the real parts. 5– 5=0 9 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 2, continued 4. Find the sum or difference of the imaginary parts. 6 i + 3 i = 9 i 10 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 2, continued 5. Find the sum of the real and imaginary parts. 0 + 9 i = 9 i 11 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 2, continued 6. Use the form of the sum to determine if it is wholly real or wholly imaginary, or if it has both a real and an imaginary part. 9 i has only an imaginary part, 9 i, so the difference is wholly imaginary. ✔ 12 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 2, continued 13 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice Example 3 Is (12 – I 20) + (– 18 – 4 i 18) wholly real or wholly imaginary, or does it have both a real and an imaginary part? 14 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 3, continued 1. Simplify any expressions containing in. Two expressions, i 20 and 4 i 18, contain in. Divide each power of i by 4 and use the remainder to simplify in. 20 ÷ 4 = 5 remainder 0, so 20 = 5 • 4 + 0. i 20 = i 5 • 4 = 1 18 ÷ 4 = 4 remainder 2, so 18 = 4 • 4 + 2. i 18 = i 4 • i 2 = – 1 15 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 3, continued Replace each occurrence of in in the expressions with the simplified versions, and replace the original expressions in the difference with the simplified expressions. i 20 = 1 4 i 18 = 4 • (– 1) = – 4 (12 – i 20) + (– 18 – 4 i 18) = (12 – 1) + [– 18 – (– 4)] 16 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 3, continued 2. Find the sum or difference of the real parts. 17 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 3, continued 3. Find the sum or difference of the imaginary parts. The expression contains no multiples of i, so there are no imaginary parts, and the multiple of i is 0: 0 i. 18 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 3, continued 4. Find the sum of the real and imaginary parts. – 3 + 0 i = – 3 19 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 3, continued 5. Use the form of the sum to determine if it is wholly real or wholly imaginary, or if it has both a real and an imaginary part. – 3 has only a real part, – 3, so the sum is wholly real. ✔ 20 4. 3. 2: Adding and Subtracting Complex Numbers

Guided Practice: Example 3, continued 21 4. 3. 2: Adding and Subtracting Complex Numbers