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Algebra Level 4

x 3 + 16 x + 175 = 0 { x }^{ 3 } + 16x + 175= 0

The equation above has roots α \alpha , β \beta and γ \gamma . Find the value of the expression below.

α γ + α β + γ β + γ α + β α + β γ \frac { \alpha }{ \gamma } +\frac { \alpha }{ \beta } +\frac { \gamma }{ \beta } +\frac { \gamma }{ \alpha } +\frac { \beta }{ \alpha } +\frac { \beta }{ \gamma }


The answer is -3.

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4 solutions

Tanishq Varshney
Apr 8, 2015

clearly sum of roots is zero.

α + β + γ = 0 \alpha+\beta+\gamma=0

α + β = γ \alpha+\beta=-\gamma ...... ( 1 ) (1)

α + γ = β \alpha+\gamma=-\beta ......... ( 2 ) (2)

β + γ = α \beta+\gamma=-\alpha ......... ( 3 ) (3)

Rearranging the given expression

α + β γ + α + γ β + β + γ α \frac{\alpha+\beta}{\gamma}+\frac{\alpha+\gamma}{\beta}+\frac{\beta+\gamma}{\alpha}

From ( 1 ) , ( 2 ) , ( 3 ) (1),(2),(3)

γ γ + β β + α α \frac{-\gamma}{\gamma}+\frac{-\beta}{\beta}+\frac{-\alpha}{\alpha}

=> 1 1 1 = 3 -1-1-1=\boxed{-3}

Nice solution! I would've done it with transformation but your method is much clever.

Kunal Verma - 6 years, 2 months ago

My method actually looks a bit longer than it is, because I explained it in a really detailed manner, but yours is indeed shorter. Nice!

Rick B - 6 years, 2 months ago

Did exact same. Overrated.

Rushikesh Joshi - 6 years, 2 months ago

Did the same way.

Niranjan Khanderia - 5 years ago
Naman Kapoor
Apr 16, 2015

My method resembles to that of tanishq but here's how I approached

For the convenience of my typing I am taking roots as a,b,c.

a+b+c=0

Summing the expression given we get

x + y z + y + x x + z + x y \frac{x+y}{z}+\frac{y+x}{x}+\frac{z+x}{y}

adding 3 and subtracting 3 we get

(x+y+z) 1 x + 1 y + 1 z \frac{1}{x}+\frac{1}{y}+\frac{1}{z} -3

Since x+y+z=0 so value of the expression is 3 \boxed{-3}

Rick B
Apr 8, 2015

Let α γ + α β + γ β + γ α + β α + β γ = S \dfrac{\alpha}{\gamma}+\dfrac{\alpha}{\beta}+\dfrac{\gamma}{\beta}+\dfrac{\gamma}{\alpha}+\dfrac{\beta}{\alpha}+\dfrac{\beta}{\gamma} = S

Multiplying S S by α β γ \alpha\beta\gamma :

α β γ S = α 2 β + α 2 γ + γ 2 α + γ 2 β + β 2 γ + β 2 α \alpha\beta\gamma S = \alpha^2\beta+\alpha^2\gamma+\gamma^2\alpha+\gamma^2\beta+\beta^2\gamma+\beta^2\alpha

= α β ( α + β ) + α γ ( α + γ ) + β γ ( β + γ ) = \alpha\beta(\alpha+\beta)+\alpha\gamma(\alpha+\gamma)+\beta\gamma(\beta+\gamma)

= α β ( α + β + γ γ ) + α γ ( α + γ + β β ) + β γ ( β + γ + α α ) =\alpha\beta(\alpha+\beta+\gamma-\gamma)+\alpha\gamma(\alpha+\gamma+\beta-\beta)+\beta\gamma(\beta+\gamma+\alpha-\alpha)

By Vieta's, α β γ = 175 \alpha\beta\gamma = -175 and α + β + γ = 0 \alpha+\beta+\gamma = 0 , so:

175 S = α β ( 0 γ ) + α γ ( 0 β ) + β γ ( 0 α ) -175S = \alpha\beta(0-\gamma)+\alpha\gamma(0-\beta)+\beta\gamma(0-\alpha)

= α β ( γ ) + α γ ( β ) + β γ ( α ) = \alpha\beta(-\gamma)+\alpha\gamma(-\beta)+\beta\gamma(-\alpha)

175 S = 3 α β γ = 525 \implies -175S = -3\alpha\beta\gamma = 525

S = 525 175 = 3 \implies S = \dfrac{525}{-175} = \boxed{-3}

I have got a shorter method

Tanishq Varshney - 6 years, 2 months ago
Ankith A Das
Apr 9, 2015

C l e a r l y t h e s u m o f r o o t s i s z e r o α + β + γ = 0 ( 1 ) 1 α + 1 β + 1 γ = ? ( 2 ) ( 1 ) × ( 2 ) ( α + β + γ ) ( 1 α + 1 β + 1 γ ) = 1 + α β + α γ + β α + 1 + β γ + γ α + γ β + 1 0 = α β + α γ + β α + β γ + γ α + γ β + 3 α β + α γ + β α + β γ + γ α + γ β = ( 3 ) Clearly\quad the\quad sum\quad of\quad roots\quad is\quad zero\\ \Rightarrow \alpha +\beta +\gamma =0\quad \quad \quad \quad \quad \quad \quad (1)\\ \frac { 1 }{ \alpha } +\frac { 1 }{ \beta } +\frac { 1 }{ \gamma } =?\quad \quad \quad \quad (2)\\ (1)\times (2)\\ \left( \alpha +\beta +\gamma \right) \left( \frac { 1 }{ \alpha } +\frac { 1 }{ \beta } +\frac { 1 }{ \gamma } \right) =1+\frac { \alpha }{ \beta } +\frac { \alpha }{ \gamma } +\frac { \beta }{ \alpha } +1+\frac { \beta }{ \gamma } +\frac { \gamma }{ \alpha } +\frac { \gamma }{ \beta } +1\\ \Rightarrow 0=\frac { \alpha }{ \beta } +\frac { \alpha }{ \gamma } +\frac { \beta }{ \alpha } +\frac { \beta }{ \gamma } +\frac { \gamma }{ \alpha } +\frac { \gamma }{ \beta } +3\\ \Rightarrow \frac { \alpha }{ \beta } +\frac { \alpha }{ \gamma } +\frac { \beta }{ \alpha } +\frac { \beta }{ \gamma } +\frac { \gamma }{ \alpha } +\frac { \gamma }{ \beta } =(-3)

Nice solution but what if the question I gave you did include a coefficient for x 2 x^{2} ? What would've you done then.

Kunal Verma - 6 years, 2 months ago

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