Grade 11 · Math
Multiply a pair of complex conjugates
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Let i2 = −1. Find (5 + 2i)(5 − 2i).
- 31
- 28
- 29
- 30
See the answer & why
29
The middle terms cancel. 52 − (2i)2 = 25 − 4(−1) = 29.
A little hint: Use the difference-of-squares identity, then replace i2 with −1.
Recognize conjugates a + bi and a − bi, which have opposite imaginary parts.
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