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How to determine the number of  real solutions of th equation f(x)=a, f(x)=1/(x-1) +...

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ggenius | Student, Undergraduate | (Level 2) Honors

Posted June 8, 2009 at 4:58 PM via web

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How to determine the number of  real solutions of th equation

f(x)=a, f(x)=1/(x-1) + 1/(x-2)+....+1/(x-2009)?

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neela | High School Teacher | (Level 3) Valedictorian

Posted June 8, 2009 at 11:51 PM (Answer #1)

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a=1/(x-1)+1/(x-2)+..1(x-2009)

Multiply by the LCM both sides and rewrite as:

P(x)=a(x-1)(x-2)(x-b)... (x-2009)

- (x-2)(x-3)--(x-2009)

-(x-1)(x-3)--(x-2009)

-(x-1)(x-2)(x-4)----(x-2009)

------------

------------

-(x-1(x-2)(x-3)---(2-2008)

 

Put x=1, x=2,x=3 suceesively and see that

p(1) =0 -(1-2)(1-3)+..+(1-2009)+0+0++0 =-(2008 -ve factors)=-ve.

p(2)=0-(2-1)((2-3)(2-4)+...+(2-2009)+0+..+0=-(one positive)(2007 -ve factor) = +ve,

Thus we can see that P(3) negative and P(4) +ve , P(5) -ve and so on and  P(n) and P(n+1) have different signs.

This indicates there is real roots between 1 and 2, between 2and 3, between 3 and 4 and so on and between 2007 and 2008.

So there are 2007 real solutions  and the other one has to be real as imaginary roots should occur in pairs for any polynomial with rational coefficients.

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