An algebra problem by Abhijith Asokan

Algebra Level 2

Let a , b , c a,b,c be the roots of the equation 2 x 3 3 x 2 + 5 x 7 = 0 { 2x }^{ 3 }-3{ x }^{ 2 }+{ 5 }x-7=0 then find 1 a + 1 b + 1 c \frac { 1 }{ a } +\frac { 1 }{ b } +\frac { 1 }{ c } . Answer is of the form x y \frac { x }{ y } where x x and y y are positive integers. Enter x + y x+y .


The answer is 12.

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3 solutions

Chew-Seong Cheong
Sep 27, 2014

Yes, since the roots are a a , b b , and c c , this means that:

( x a ) ( x b ) ( x c ) = x 3 3 2 x 2 + 5 2 x 7 2 (x-a)(x-b)(x-c) = x^3 - \frac{3}{2}x^2 + \frac{5}{2}x - \frac{7}{2}

x 3 ( a + b + c ) x 2 + ( a b + b c + c a ) x a b c = x 3 3 2 x 2 + 5 2 x 7 2 \Rightarrow x^3 -(a+b+c)x^2 + (ab+bc+ca)x - abc = x^3 - \frac{3}{2}x^2 + \frac{5}{2} x- \frac{7}{2}

a b + b c + c a = 5 2 a b c = 7 2 \Rightarrow ab+bc+ca = \frac{5}{2} \quad abc = \frac{7}{2}

Since 1 a + 1 b + 1 c = a b + b c + c a a b c = 5 3 7 2 = 5 7 = x y \dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c} = \dfrac{ab + bc +ca}{abc} = \dfrac {\frac{5}{3}}{\frac{7}{2}} = \dfrac{5}{7}= \dfrac{x}{y}

Therefore, x + y = 5 + 7 = 12 x +y = 5 +7 = \boxed{12}

Abhijith Asokan
Sep 18, 2014

1 a + 1 b + 1 c = b c + a c + a b a b c = C / A D / A = 5 / 2 ( 7 ) / 2 \frac { 1 }{ a } +\frac { 1 }{ b } +\frac { 1 }{ c } =\frac { bc+ac+ab }{ abc } =\frac { C/A }{ -D/A } =\frac { 5/2 }{ -(-7)/2 } n=5,m=7 n+m=12

http://www4a.wolframalpha.com/Calculate/MSP/MSP672020496i75b610hh8100003ig243cgcg82h6c7?MSPStoreType=image/gif&s=40&w=325.&h=95. is the only available real solution for the equation i.e. approximately 1.4455

aaron paul - 6 years, 8 months ago

can you tell me what is the mistake in this can we write the equation as x(2x^2 - 3x +5) = 7 * 1 and by comparing we get x=1 or x = -1/2 or x=2 therefore 1/a + 1/b +1/c =-1/2 therefore x +y =1

U Z - 6 years, 8 months ago

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Can you be a little bit more clear about what you mean by 'comparing'?

Mursalin Habib - 6 years, 8 months ago

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comparing 7 with the quadratic equation and 1 with x

U Z - 6 years, 8 months ago

That's exactly how I did it.

William Isoroku - 6 years, 8 months ago
Aravind M
Oct 3, 2014

For any equation, (quad, tri., biquad,..) for an eq. with roots a1,b1,c1,d1......, the eq with roots 1/a1, 1/a2, 1/a3.... can be obtained by just reversing the coefficients..

In this case coeff are 2, -3 ,+5, -7. .. reversing coeffs, we have the new eq: -7(x^3) +5(x^2) -3(x)=2=0 this equations sum= 1/a +1/b + 1/c = -(-5/7)=5/7= x+y= 5+7==12..

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