30 COMPLEX NUMBERS The real and imaginary components IN THE LESSON ON RADICALS, we saw how to solve any equation in this form:
But x² + 1 = 0 has a solution.
i = The complex number i is purely algebraic. That is, we call it a "number" because it will obey all the rules we normally associate with a number. We may add it, subtract it, multiply it, and so on. The basic algebraic property of i is the following: i² = −1 Example 1. 3i· 4i = 12i² = 12(−1) = −12. Example 2. −5i· 6i = −30i² = 30. We can see, then, that the factor i² changes the sign of a product. Problem 1. Evaluate the following. To see the answer, pass your mouse over the colored area.
Negative radicand If a radicand is negative --
-- then we can simplify it as follows:
Problem 2. Express each of the following in terms of i.
Powers of i Let us begin with i0, which is 1. (Any number with exponent 0 is 1.) Each power of i can be obtained from the previous power by multiplying it by i. We have:
And we are back at 1 -- the cycle of powers will repeat! Any power of i will be either 1, i, −1, or −i -- according to the remainder upon dividing the exponent n by 4. Examples 4 .
Note: Even powers of i will be either 1 or −1, according as the exponent is a multiple of 4 or 2 more than a multiple of 4. While odd powers will be either i or −i. Problem 3. Evaluate each power of i.
Algebra with complex numbers Complex numbers follow the same rules as real numbers. For example, to multiply (2 + 3i)(2 − 3i) the student should recognize the form (a + b)(a − b) -- which will produce the difference of two squares. Therefore,
Again, the factor i² changes the sign of the term. Problem 4. Multiply. a) (1 + i
Problem 5. (x + 1 + 3i)(x + 1 − 3i) a) What form will that produce? The difference of two squares. b) Multiply out.
The real and imaginary components Here is the standard form of a complex number: a + bi, where both a and b are real. For example, 3 + 2i. a -- that is, 3 in the example -- is called the real component (or the real part). b (2 in the example) is called the imaginary component (or the imaginary part). Again, the components are real. Problem 6. Name the real component a and the imaginary component b.
Complex conjugates The complex conjugate of a + bi is a − bi. The main point about a conjugate pair is that when they are multiplied -- (a + bi)(a − bi) -- a positive real number is produced. For, that form is the difference of two squares: (a + bi)(a − bi) = a² − b²i² = a² + b² The product of a conjugate pair Problem 7. Calculate the positive real number that results from multiplying each number with its complex conjugate. a) 2 + 3i. (2 + 3i)(2 − 3i) = 2² + 3² = 4 + 9 = 13 b) 3 − i c) u + iv. (u + iv)(u − iv) = u² + v² d) 1 + i. (1 + i)(1 − i) = 1² + 1² = 2 e) −i. (−i)(i) = −i² = 1 ![]() Next Lesson: Rectangular coordinates Please make a donation to keep TheMathPage online. Copyright © 2001-2007 Lawrence Spector Questions or comments? E-mail: themathpage@nyc.rr.com |