α , β , γ ∑ 1 − α ( 1 + α ) 2
Given that α , β and γ are the roots of x 3 − x − 1 = 0 , then find the value of above expression.
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Since you're doing a substitution, can we directly use u = 1 − x ( 1 + x ) 2 ? Why, or why not?
n = { α , β , γ } ∑ 1 − n ( 1 + n ) 2 = 1 − α ( 1 + α ) 2 + 1 − β ( 1 + β ) 2 + 1 − γ ( 1 + γ ) 2
From Vieta's formula, we know that
α + β + γ = 0 α β + α γ + β γ = − 1 α β γ = 1
Now, to simplify our calculations, we substitute α = a + 1 , β = b + 1 and γ = c + 1 into the expression and the equations
The expression becomes:
− a ( a + 2 ) 2 + − b ( b + 2 ) 2 + − c ( c + 2 ) 2 = − ( a b c b c ( a 2 + 4 a + 4 ) + a c ( b 2 + 4 b + 4 ) + a b ( c 2 + 4 c + 4 ) ) = − ( a b c a 2 b c + a b 2 c + a b c 2 + 1 2 a b c + 4 a b + 4 a c + 4 b c ) = − ( a b c a b c ( a + b + c ) + 1 2 a b c + 4 ( a b + a c + b c ) )
The equations become (simplify yourself, I'm too lazy to type out all the steps):
a + 1 + b + 1 + c + 1 = 0 ⟹ a + b + c = − 3 ( a + 1 ) ( b + 1 ) + ( a + 1 ) ( c + 1 ) + ( b + 1 ) ( c + 1 ) = − 1 ⟹ a b + a c + b c = 2 ( a + 1 ) ( b + 1 ) ( c + 1 ) = 1 ⟹ a b c = 1
Substitute these values into the expression to get the answer:
− ( a b c a b c ( a + b + c ) + 1 2 a b c + 4 ( a b + a c + b c ) ) = − ( 1 1 ( − 3 ) + 1 2 ( 1 ) + 4 ( 2 ) ) = − 1 7
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First of all, note that x = 1 is not a root to the equation x 3 − x − 1 = 0 , so, the value of given expression is evaluate-able.
Notice that 1 − α ( 1 + α ) 2 = 1 − α ( 1 − α ) 2 + 4 α = 1 − α + 4 ( 1 − α α ) . Similarly, 1 − β ( 1 + β ) 2 = 1 − β + 4 ( 1 − β β ) and 1 − γ γ 2 = 1 − γ + 4 ( 1 − γ γ ) .
So, ∑ cyc 1 − α ( 1 + α ) 2 = 3 − ( α + β + γ ) + 4 ( 1 − α α + 1 − β β + 1 − γ γ ) .
Now, replacing x by 1 + x x in the equation x 3 − x + 1 = 0 followed by appropriate simplifications gives x 3 + 5 x 2 + 4 x + 1 = 0 . Clearly, this equation has roots of the form 1 − α α . Now, Viete's Theorem, 1 − α α + 1 − β β + 1 − γ γ = − 5 . So, ∑ cyc 1 − α ( 1 + α ) 2 = 3 − 0 + 4 ⋅ ( − 5 ) = − 1 7 .