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Please help on puzzle #2

Please help on puzzle #2-example-1

1 Answer

3 votes

Answer:

(2 +4i)(5 -6i) . . . or . . . (4 -2i)(6 +5i)

Explanation:

The product of two complex numbers is ...

(a +bi)(c +di) = (ac -bd) +(bc +ad)i

So, we're looking for pairs of numbers that can be combined in different ways to give 34 and 8. The numbers we found (by trial and error) are ...

2, 4, 5, 6

where 4*6 +2*5 = 34 and 4*5 -2*6 = 8. Because of the effect if i^2 on the sign, we need to have the imaginary parts have opposite signs.

Each of the solutions shown above is representative of 4 solutions. For example, for the first one, you could have ...

(2 +4i)(5 -6i) = (2·5 +4·6) + (4·5 +2(-6))i = 34 +8i

(5 -6i)(2 +4i) = (5·2 +6·4) + (-6·2 +5·4) = 34 +8i . . . . . order of factors swapped

(-2 -4i)(-5 +6i) = ((-2)(-5) -(-4)(6)) + ((-4)(-5) +(-2)(6))i = 34 +8i . . . . both factors in the first solution negated

(-5 +6i)(-2 -4i) = ((-5)(-2) -(6)(-4)) +(6(-2) +(-5)(-4))i = 34 +8i . . . . factors swapped and negated

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Likewise, the second shown solution above is representative of 4 solutions.

Possible solutions are ...

  • (2 +4i)(5 -6i)
  • (4 -2i)(6 +5i)

with sign and order variations.

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Comment on trial and error

Actually, we did an exhaustive search of the 441 products of single-digit numbers [-9, 9] to see which pairs of them differed by 34. Then, among those, we looked for product pairs that added to 8. In the end, we found the 8 solutions described above.

User Dean Wild
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