A Complex Problem

Algebra Level 5

Let a , b , c a, b, c be 3 complex numbers such that a = b = c = 1 |a| = |b| = |c| = 1 and a 2 b c + b 2 a c + c 2 a b + 1 = 0 \dfrac{a^2}{bc}+ \dfrac{b^2}{ac} + \dfrac{c^2}{ab} + 1 = 0 . If a + b + c |a + b + c| can take values that equal p p and q q , then find the value of p + q p+q .


The answer is 3.

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2 solutions

I believe the answer may be wrong, or my solution may have a problem. Here it goes:

The magnitude of a , b , c a, b, c is 1 1 so, a a = b b = c c = 1 \overline{a}a = \overline{b}b = \overline{c}c = 1 \Rightarrow

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 |a+b+c|^2=(a+b+c)\overline{(a+b+c)} = ... = 3 + a\overline{b} +a\overline{c} + b\overline{a} + b\overline{c} + c\overline{a} + c\overline{b} = X ^2

Using the initial equalities we get:

X 2 = 3 + a b + a c + b a + b c + c a + c b = 3 + 3 3 a 2 c + a 2 b + b 2 c + b a + c 2 b + c 2 a a b c = X^2= 3 + \frac {a}{b} +\frac {a}{c} +\frac {b}{a} +\frac {b}{c} + \frac {c}{a} +\frac {c}{b} = 3 + \frac{3}{3} \cdot \frac{a^2c +a^2b +b^2c +b^a +c^2b +c^2a}{abc} =

3 + ( a + b + c ) 3 ( a 3 + b 3 + c 3 + 6 a b c ) 3 a b c = 3 +\frac{(a+b+c)^3 - (a^3+b^3+c^3+6abc)}{3abc} =

3 + ( a + b + c ) 3 5 a b c 3 a b c = 3 + \frac{(a+b+c)^3-5abc}{3abc} =

4 3 + ( a + b + c ) 3 3 a b c \frac {4}{3}+ \frac{(a+b+c)^3}{3abc}

But, ( a + b + c ) 3 3 a b c \frac{(a+b+c)^3}{3abc} should be a real number so,

( a + b + c ) 3 3 a b c = ( a + b + c ) 3 3 a b c = ( a + b + c ) 3 3 = X 3 3 \frac{(a+b+c)^3}{3abc} = \frac{|(a+b+c)|^3}{3|abc|} =\frac{|(a+b+c)|^3}{3} = \frac{X^3}{3}

We have:

X 2 = 4 3 + X 3 3 X^2 = \frac {4}{3} + \frac{X^3}{3}

Which gives X = 2 , 1 X= 2,-1

The grader accepted 3 as an answer but we cannot accept the 1 -1 value as magnitude.

If you find an error in my solution please, tell me to edit it.

:)

When x is ensured as real number , we still don't have x=|x|

Teerawat Tri'Niset - 4 years, 9 months ago
Lu Chee Ket
Dec 17, 2015

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 ^\circ and 180 ^\circ are of equal place and therefore only 180 ^\circ can be taken as sole definition.

1
2
3
4
5
6
7
0   1
-60 0.5-0.866025403784439i
60  0.5+0.866025403784439i

0   1
180 -1
180 -1

Where magnitudes of 2 and 1 can be there as p and q, we don't take -60 ^\circ and 60 ^\circ as ordered in their set. Just try your own ways to evaluate the two sets to your favorites.

Answer: 3 \boxed{3}

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