An algebra problem by swapnil rajawat

Algebra Level 4

For all negative reals c c , let α c \alpha_c and β c \beta_c be the roots of the equation 2 x 2 + 6 x + c = 0 2x^2 + 6x + c = 0 . What is the smallest integer that must be greater than

α c β c + β c α c ? \frac{ \alpha_c} { \beta_c } + \frac{ \beta_c } { \alpha_c}?


The answer is -2.

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2 solutions

Maharnab Mitra
May 7, 2014

For simplicity in writing, I'm referring to the roots as α \alpha and β \beta .

α + β = 3 \alpha+ \beta =-3 and α β = c 2 \alpha \beta =\frac{c}{2} ,

So, α β + β α = α 2 + β 2 α β = ( α + β ) 2 2 α β α β = 2 ( 9 c ) c = 18 c 2 \frac{\alpha}{\beta}+\frac{\beta}{\alpha}=\frac{\alpha^2 +\beta^2}{\alpha \beta} =\frac{(\alpha + \beta)^2 - 2\alpha \beta}{\alpha \beta} =\frac{2(9-c)}{c}=\frac{18}{c} -2

Consider y = 18 c 2 y=\frac{18}{c} -2 which we have to maximize for c < 0 c<0 . It is the graph of a rectangular hyperbola in the fourth quadrant. After tracing the graph, you will find that the line y = 2 y=-2 is an asymptote to the curve and the curve lies entirely below it. Thus, the smallest integer greater than y y is 2 \boxed{-2}

Why must c be < 0

Star Light - 7 years, 1 month ago

Ok, got it, it is given in the question

Star Light - 7 years, 1 month ago
Kanthi Deep
May 5, 2014

If one root b<0 then c (constant term )must be negative because the coefficient of X is positive hence another root a must be positive So a/b is negative.... Then by inequality "if x<0 then x+1/x<2" so a/b +b /a <2

How did you arrive to this inequality?

swapnil rajawat - 7 years, 1 month ago

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I've edited your question for clarity. Can you review it for accuracy?

Calvin Lin Staff - 7 years, 1 month ago

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thanks for reviewing this question is alright now.

swapnil rajawat - 7 years, 1 month ago

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