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Use the imaginary unit i to write complex numbers.
Add, subtract, and multiply complex numbers.
Use complex conjugates to write the quotient of two complex numbers in standard form.
Perform operations with square roots of negative numbers
Solve quadratic equations with complex imaginary solutions
COMPLEX NUMBERS
Objectives
R
Real Numbers R
Rational Numbers Q
Integers Z
Whole numbers W
Natural Numbers N
IrrationalNumbers
Q -bar
Complex Numbers C
Imaginary Numbers i
What is an imaginary number?
It is a tool to solve an equation and was invented to solve quadratic equations of the form .
. It has been used to solve
equations for the last 200 years or so.
“Imaginary” is just a name, imaginary do indeed exist; they are numbers.
Previously, when we encountered square roots of negative numbers in solving equations, we would say “no real solution” or “not a real number”.
The Imaginary Unit i
Complex Numbers & Imaginary Numbers
a + bi represents the set of complex numbers, where a and b are real numbers and i is the imaginary part.
a + bi is the standard form of a complex number. The real number a is written first, followed by a real number b multiplied by i. The imaginary unit i always follows the real number b, unless b is a radical. Example:
If b is a radical, then write i before the radical.
Adding and Subtracting Complex Numbers
(5 − 11i) + (7 + 4i)
Simplify and treat the i like a variable.
= 5 − 11i + 7 + 4i
= (5 + 7) + (− 11i + 4i)
= 12 − 7i Standard form
Adding and Subtracting Complex Numbers
(− 5 + i) − (− 11 − 6i)
= − 5 + i + 11 + 6i
= − 5 + 11 + i + 6i
= 6 + 7i
(5 – 2i) + (3 + 3i)
5+3 −2 𝑖+3 𝑖
𝟖+𝒊
(2 + 6i) − (12 − i)
2+6 𝑖− 12+𝑖
2 −12+6 𝑖+𝑖
−𝟏𝟎+𝟕 𝒊
Multiplying Complex Numbers
4i (3 − 5i)
4 𝑖 (3 ) − 4 𝑖(5 𝑖)
12 𝑖− 20 𝑖2 𝒊𝟐=−𝟏
12 𝑖− 20(− 1)
12 𝑖+20
𝟐𝟎+𝟏𝟐𝒊 Standard form
Multiplying Complex Numbers
(7 − 3i )( − 2 − 5i) use FOIL
−14 −35 𝑖+6 𝑖+15 𝑖2
−14 −29 𝑖+15 (−1)
𝒊𝟐=−𝟏
−14 −29 𝑖−15
−𝟐𝟗−𝟐𝟗𝒊 Standard form
7i (2 − 9i)
7 𝑖 (2 )+7 𝑖(−9 𝑖)
14 𝑖−63 𝑖2 𝒊𝟐=−𝟏
14 𝑖−63 (−1)
14 𝑖+63
𝟔𝟑+𝟏𝟒𝒊 Standard form
(5 + 4i)(6 − 7i)
30 − 35 𝑖+24 𝑖−28 𝑖2 𝒊𝟐=−𝟏
30−11 𝑖− 28(−1)
30−11 𝑖+28
30+28−11 𝑖
𝟓𝟖−𝟏𝟏 𝒊 Standard form
Complex Conjugates
The complex conjugate of the number a + bi is a − bi.
Example: the complex conjugate of is
The complex conjugate of the number a − bi is a + bi.
Example: the complex conjugate of is
Complex Conjugates
When we multiply the complex conjugates together, we get a real number.
(a + bi) (a − bi) = a² + b²
Example:
Complex ConjugatesWhen we multiply the complex conjugates together, we get a real number.
(a − bi) (a + bi) = a² + b²
Example:
Using Complex Conjugates to Divide Complex Numbers
Divide and express the result in standard form:
7 + 4i2 − 5i
The complex conjugate of the denominator is 2 + 5i.Multiply both the numerator and the denominator by the complex conjugate.
Using Complex Conjugates to Divide Complex Numbers
x (14+35 𝑖+8 𝑖+20 𝑖2 )4 −25 𝑖2¿
14+43 𝑖+20 (− 1 )4 − 25 (−1 )
14+43 𝑖−204+25¿
−6+43 𝑖29 ¿−
𝟔𝟐𝟗
+𝟒𝟑𝟐𝟗
𝒊
Divide and express the result in standard form:
5 + 4i4 − i
Roots of Negative Numbers
= • i =
= 4 • i
Operations Involving Square Roots of Negative Numbers
See examples on page 282.
The complex-number system is used to find zeros of functions that are not real numbers.
When looking at a graph of a function, if the graph does not cross the x-axis, it has no real-number zeros.
A Quadratic Equation with Imaginary Solutions
See example on page 283.