Divisible by this year? (Part 8: Get JINXED! Again.)

Algebra Level 5

If you randomly picked one root of the function p ( x ) = ( x 4 + 1989 x 3 8162742 x 2 + 202809800 x ) ( e 2014 x 1 ) [ ln ( x 10069 ) ] p(x)=(x^4 + 1989x^3 -8162742x^2+202809800x)(e^{2014x}-1)[\ln { (x-10069) } ] and the function is from R \mathbb{R} to R \mathbb{R} , what is the probability that the root that you picked is divisible by 2014 2014 ?

More at Divisible by this year???

100 100 % None of the above 83 1 3 83 \frac{1}{3} % 80 80 %

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1 solution

Rindell Mabunga
Dec 15, 2014

Let f ( x ) = x 4 + 1989 x 3 8162747 x 2 + 202809800 x f(x) = x^4 + 1989x^3 -8162747x^2+202809800x

also g ( x ) = e 2014 x 1 g(x) = e^{2014x}-1

and h ( x ) = ln ( x 10069 ) h(x) = \ln { (x-10069) }

We must equate all these factors to 0 0 .

Factoring f ( x ) f(x) we will get

f ( x ) = x ( x 25 ) ( x 2014 ) ( x + 4028 ) = 0 f(x) = x(x - 25)(x - 2014)(x + 4028) = 0

Thus the roots of f ( x ) f(x) are x = 0 , 25 , 2014 , 4028 x = 0, 25, 2014, -4028

Equating g ( x ) = 0 g(x) = 0 gives

g ( x ) = 0 g(x) = 0

e 2014 x 1 = 0 e^{2014x}-1 = 0

e 2014 x = 1 e^{2014x} = 1

e 2014 x = e 0 e^{2014x} = e^0

2014 x = 0 2014x = 0

x = 0 x = 0

Equating h ( x ) = 0 h(x)=0 gives

h ( x ) = 0 h(x) = 0

ln ( x 10069 ) = 0 \ln { (x-10069) } = 0

ln ( x 10069 ) = ln 1 \ln { (x-10069) } = \ln1

x 10069 = 1 x - 10069 = 1

x = 10070 x = 10070 .

These are not yet the roots of p ( x ) p(x) . To get the roots of p ( x ) p(x) , we must SUBSTITUTE them back to the original equation.

Only x = 10070 x = 10070 satisfies p ( x ) = 0 p(x) = 0 since the other values of x x makes the natural logarithm part undefined.

Since 10070 10070 is divisible by 2014 2014 , the probability will be 100 100 %

You have to specify that the function is from R \mathbb{R} to R \mathbb{R} . Also, it turns out that x ( x 25 ) ( x 2014 ) ( x + 4028 ) = x 4 + 2009 x 3 8122462 x 2 + 40561960 x x(x-25)(x-2014)(x+4028) = x^{4}+2009x^{3}-8122462 x^{2}+40561960x .

Jake Lai - 6 years, 6 months ago

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But 25 × 2014 × 4028 = 202809800 -25 \times -2014 \times 4028 = 202809800 and not 40561960 40561960

25 2014 + 4028 = 1989 -25-2014+4028=1989 not 2009 2009

and ( 25 × 2014 ) + ( 2014 × 4028 ) + ( 4028 × 25 ) = 8162747 (-25 \times -2014) + (-2014 \times 4028) + (4028\times-25) = -8162747 not 8122462 -8122462

So its correct.

Julian Poon - 6 years, 5 months ago

No Sorry but you made a mistake. The roots of the polynomial that you made are 5, 2014, and -4028

Rindell Mabunga - 6 years, 6 months ago

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Yeah, that's my point. Your polynomial's roots are slightly off from 5, 2014 and -4028.

Jake Lai - 6 years, 6 months ago

But ok I will add R to R

Rindell Mabunga - 6 years, 6 months ago

Mother of mercy, that was sneaky!

James Wilson - 3 years, 7 months ago

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