Engineering
Mathematics
Conditional Probability
Question

Let ω be a complex cube root of unity with ω ≠ 1. A fair die is thrown three times. If r1, rand r3 are the numbers obtained on the die then the probability that ωr1+ωr2+ωr3=0 is

118

19

29

136

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Solution
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r1, r2, r∈ {1, 2, 3, 4, 5, 6}
r1, r2, r3 are if the form 3k 3k + 1, 3k + 2
Required probability 3!×2C1×2C1×2C16×6×6=6×8216=29

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