College Algebra / Unit 4 Alg 187 short lesson, about 20 seconds
The Complex Conjugate
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The idea
The complex conjugate of a + bi is a − bi; a complex number times its conjugate is the real number a² + b².
In practice
(3 + 2i)(3 − 2i) = 9 − 4i² = 9 + 4 = 13 — conjugate binomials with i inside, and the middle terms cancel.
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