It may be complex

Algebra Level 4

Let a a , b b and c c be roots of the polynomial P ( x ) = x 3 2 x 2 + 3 x 4 P(x)=x^3-2x^2+3x-4 . Determine

( a + 1 a ) ( b + 1 b ) ( c + 1 c ) . \large \left(a+\dfrac{1}{a}\right)\left(b+\dfrac{1}{b}\right)\left(c+\dfrac{1}{c}\right).


The answer is 2.

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2 solutions

x 3 2 x 2 + 3 x 4 x^3-2x^2+3x-4

By Vieta's formula.
a + b + c = 2 a+b+c=2
a b + b c + c a = 3 ab+bc+ca=3
a b c = 4 abc=4


( a + 1 a ) × ( b + 1 b ) × ( c + 1 c ) \Rightarrow \left(a+\frac{1}{a}\right)×\left(b+\frac{1}{b}\right)×\left(c+\frac{1}{c}\right)

= ( a b c ) 2 + [ ( a b ) 2 + ( b c ) 2 + ( c a ) 2 ] + [ a 2 + b 2 + c 2 ] + 1 ( a b c ) =\frac{(abc)^2+\color{#20A900}{\left[(ab)^2+(bc)^2+(ca)^2\right]}+\color{#EC7300}{\left[a^2+b^2+c^2\right]}+1}{(abc)}

= ( a b c ) 2 + [ ( a b + b c + c a ) 2 2 [ ( a b c ) ( a + b + c ) ] ] + [ ( a + b + c ) 2 2 ( a b + b c + c a ) ] + 1 ( a b c ) =\frac{(abc)^2+\color{#20A900}{\left[(ab+bc+ca)^2-2[(abc)(a+b+c)]\right]}+\color{#EC7300}{\left[(a+b+c)^2-2(ab+bc+ca)\right]}+1}{(abc)}

= ( 4 ) 2 + [ ( 3 ) 2 2 [ ( 4 ) × ( 2 ) ] ] + [ ( 2 ) 2 2 × ( 3 ) ] + 1 ( 4 ) =\frac{(4)^2+\color{#20A900}{\left[(3)^2-2[(4)×(2)]\right]}+\color{#EC7300}{[(2)^2-2×(3)]}+1}{(4)}

= 16 7 2 + 1 4 =\frac{16-\color{#20A900}{7}-\color{#EC7300}{2}+1}{4}

= 8 4 =\frac{8}{4}

= 2 =\boxed{2}

Thiago Bersch
Jun 27, 2016

just put x=i and x=-i (i-a)(-i-a)=-(a²+1) Doing this for P(i)P(-i) = 8, and abc=4 (a²+1)(b²+1)(c²+1)/abc = 8/4 = 2

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