Let a , b , c be 3 complex numbers such that ∣ a ∣ = ∣ b ∣ = ∣ c ∣ = 1 and b c a 2 + a c b 2 + a b c 2 + 1 = 0 . If ∣ a + b + c ∣ can take values that equal p and q , then find the value of p + q .
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When x is ensured as real number , we still don't have x=|x|
For magnitudes of 1, only angles are considered.
Satisfied p {0, -60, 60} and q {0, 180, 180} makes | a + b + c | of | 2 | and | -1 | respectively for 2 + 1 = 3.
Note that -180 ∘ and 180 ∘ are of equal place and therefore only 180 ∘ can be taken as sole definition.
1 2 3 4 5 6 7 |
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Where magnitudes of 2 and 1 can be there as p and q, we don't take -60 ∘ and 60 ∘ as ordered in their set. Just try your own ways to evaluate the two sets to your favorites.
Answer: 3
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I believe the answer may be wrong, or my solution may have a problem. Here it goes:
The magnitude of a , b , c is 1 so, a a = b b = c c = 1 ⇒
We also have ∣ a + b + c ∣ 2 = ( a + b + c ) ( a + b + c ) = . . . = 3 + a b + a c + b a + b c + c a + c b = X 2
Using the initial equalities we get:
X 2 = 3 + b a + c a + a b + c b + a c + b c = 3 + 3 3 ⋅ a b c a 2 c + a 2 b + b 2 c + b a + c 2 b + c 2 a =
3 + 3 a b c ( a + b + c ) 3 − ( a 3 + b 3 + c 3 + 6 a b c ) =
3 + 3 a b c ( a + b + c ) 3 − 5 a b c =
3 4 + 3 a b c ( a + b + c ) 3
But, 3 a b c ( a + b + c ) 3 should be a real number so,
3 a b c ( a + b + c ) 3 = 3 ∣ a b c ∣ ∣ ( a + b + c ) ∣ 3 = 3 ∣ ( a + b + c ) ∣ 3 = 3 X 3
We have:
X 2 = 3 4 + 3 X 3
Which gives X = 2 , − 1
The grader accepted 3 as an answer but we cannot accept the − 1 value as magnitude.
If you find an error in my solution please, tell me to edit it.
:)