A Different Approach!

Algebra Level 4

if a,b,c are the roots of the equation x 3 x 1 = 0 { x }^{ 3 }-x-1=0 find 1 a + 1 + 1 b + 1 + 1 c + 1 \frac { 1 }{ a+1 } +\frac { 1 }{ b+1 } +\frac { 1 }{ c+1 }


The answer is 2.

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2 solutions

Drex Beckman
Dec 27, 2015

Using Descarte's rule of signs, we see that x 3 x 1 x^{3}-x-1 has 1 sign change, therefore one real solution. Trying to solve via synthetic division gives a remainder, so we need another method to solve it. We rewrite the equation with an x-squared term: x 3 + 0 x 2 x 1 x^{3}+0x^{2}-x-1 Since we cannot solve through factoring: x = 1 2 + ( 1 3 ) 3 + ( 1 2 ) 2 3 + 1 2 ( 1 3 ) 3 + ( 1 2 ) 2 3 ( 1 3 ) 0 1.65 x=\sqrt[3]{\frac{1}{2}+\sqrt{(\frac{-1}{3})^{3}+(\frac{1}{2})^{2}}}\hspace{2mm}+\sqrt[3]{\frac{1}{2}-\sqrt{(\frac{-1}{3})^{3}+(\frac{1}{2})^{2}}}\hspace{2mm}-(\frac{1}{3})\cdot 0\approx 1.65 Now we add: 1 2.65 + 1 1 + 0 + 1 1 + 0 2.3 \frac{1}{2.65}+\frac{1}{1+0}+\frac{1}{1+0}\approx 2.3 So we round down, and get a result of 2.

Can you explain how did you got x x ?

Aditya Sky - 4 years ago

I didn't get it !!

harsh soni - 3 years, 11 months ago

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Actually, we found out the roots of the cubic first and then substituted their values in the given expression.

Aditya Sky - 3 years, 10 months ago
Asif Mujawar
Sep 14, 2018

a+b+c=0 ab+bc+ac=-1 abc=1 Putting above values in simplified expression we get 2

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