2 x 3 + x 2 − 7 = 0
If α , β , γ are the roots of the above equation, evaluate
∣ ∣ ∣ ∣ ∣ ∣ cyc ∑ ( β α + α β ) ∣ ∣ ∣ ∣ ∣ ∣
Note
∣ z ∣ is the absolute value of z
cyc ∑ means cyclic summation.
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Is it possible to generalize this?
Given that α = a , β = b , γ = c
Using Vieta's formulas you obtain these three equations:
a b c = 7 / 2
a b + b c + a c = 0
a + b + c = − 1 / 2
From the first equation we obtain that:
a b = 7 / 2 c
b c = 7 / 2 a
a c = 7 / 2 b
This means that:
1 / a + 1 / b + 1 / c = 0
The solution is given by:
( 1 / a + 1 / b + 1 / c ) ∗ ( a + b + c ) = 0
a / b + b / a + a / c + c / a + b / c + c / b + a / a + b / b + c / c = 0
( a / b + b / a ) + ( a / c + c / a ) + ( b / c + c / b ) = − 3
∣ − 3 ∣ = 3
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Note that α β + β γ + γ α = 0
∣ ∣ ∣ ∣ ∣ ∣ cyc ∑ ( β α + α β ) ∣ ∣ ∣ ∣ ∣ ∣ = ∣ ∣ ∣ α β γ α 2 β + α 2 γ + β 2 α + β 2 γ + γ 2 α + γ 2 β ∣ ∣ ∣
= ∣ ∣ ∣ α β γ ( α β + β γ + γ α ) ( α + β + γ ) − 3 α β γ ∣ ∣ ∣
= ∣ ∣ ∣ α β γ − 3 α β γ ∣ ∣ ∣
= 3