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Algebra Level 3

If x 2 + 6 x + 19 = 0 x^2 + 6x +19 =0 and a x 2 + b x + c = 0 ax^2 +bx +c =0 have a common root and a , b a,b and c c are positive integers, find the minimum value of a + b + c a+b+c .

9 19 25 26

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3 solutions

Determinant of given quadratic = 6 2 4 × 19 = 36 76 = 40 < 0 6^{2} - 4 \times 19 = 36-76 = -40 < 0
Thus, the roots of given quadratic are complex.

Hence, the required quadratic has one root in common, it also has one complex root. But, since complex root occur in pairs, the required quadratic has both roots in common with the given quadratic. Thus, the co-efficients are in proportion.

a 1 = b 6 = c 19 = α \therefore \dfrac{a}{1} = \dfrac{b}{6} = \dfrac{c}{19} = \alpha

a + b + c = 26 × α \therefore a + b + c = 26 \times \alpha

For minimum integral values, of a,b,c

α = 1 \alpha = 1

M i n i m u m ( a + b + c ) = 26 \therefore Minimum \left(a + b + c \right) = 26

Pulkit Gupta
Dec 22, 2015

Let the common root be k.

Then the two quadratics can be written in terms of k as k 2 + 6 k + 19 \large k^2 + 6k + 19 & a k 2 + b k + c \large ak^2 + bk + c

Subtract the two quadratics to obtain ( a 1 ) k 2 + ( b 6 ) k + ( c 19 ) \large (a-1)k^2 + (b-6)k + (c-19) .

Now clearly for k to be unique, discriminant has to be equal to zero . This gives the value of a , b & c as 6,19 & 1.

Or, the given quadratic has complex roots. So the required quadratic has to have both roots common, meaning the co-efficients are in proportion.

A Former Brilliant Member - 5 years, 5 months ago

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Indeed. I overlooked that.

Post it as a solution :)

Pulkit Gupta - 5 years, 5 months ago

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Done. Hope it's good enough. :)

A Former Brilliant Member - 5 years, 5 months ago
Lu Chee Ket
Dec 22, 2015

Take no knowledge of theory of equation to guess for what the author wanted,

( x + 3 ) 2 + 10 = 0 (x + 3)^2 + 10 = 0

x = 3 ± 10 x = - 3 \pm \sqrt{-10} { ± \pm here stands for plus O r Or minus.}

a ( 3 ± 10 ) 2 + b ( 3 ± 10 ) + c = 0 a (- 3 \pm \sqrt{-10})^2 + b (- 3 \pm \sqrt{-10}) + c = 0

a ( 1 ± 6 10 ) + b ( 3 ± 10 ) + c = 0 a (- 1 \pm -6 \sqrt{-10}) + b (- 3 \pm \sqrt{-10}) + c = 0

We obtain two equations namely - a - 3 b + c = 0 and 6 a - b = 0.

Hence, a + b + c = a + 6 a + 19 a = 26 a = 26 with a = 1 for a minimum value wanted.

Answer: 26 \boxed{26}

Wrong. You didn't show that 26 is minimum.

Pi Han Goh - 5 years, 5 months ago

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