Why Are There So Many Powers?

Algebra Level 4

For a function f ( x ) f(x) such that

x 4 ( f ( x ) ) 4 + x 2 = 3 x 3 ( f ( x ) ) 2 , x^4\left(f(x)\right)^4+x^2=3x^3\left(f(x)\right)^2,

let the largest possible value of f ( 2014 ) f(2014) over all possible functions f ( x ) f(x) be M . M. Find 1000 M . \lfloor 1000M\rfloor .


The answer is 36.

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1 solution

Jeffery Li
Jun 9, 2014

Let's change x 4 f ( x ) 4 + x 2 = 3 x 3 f ( x ) 2 x^4f(x)^4+x^2=3x^3f(x)^2 into a more reasonable equation.

First, move 3 x 3 f ( x ) 2 3x^3f(x)^2 to the LHS to get x 4 f ( x ) 4 3 x 3 f ( x ) 2 + x 2 = 0 x^4f(x)^4-3x^3f(x)^2+x^2=0 .

Then, divide by x 2 x^2 to get x 2 f ( x ) 4 3 x f ( x ) 2 + 1 = 0 x^2f(x)^4-3xf(x)^2+1=0 .

Then, solving for x f ( x ) 2 xf(x)^2 using the quadratic formula, we get x f ( x ) 2 = 3 ± 5 2 xf(x)^2=\frac{3\pm\sqrt{5}}{2} . Substituting x = 2014 x=2014 , we get 2014 f ( 2014 ) 2 = 3 ± 5 2 2014f(2014)^2=\frac{3\pm\sqrt{5}}{2} .

Next, we divide both sides by 2014 to get f ( 2014 ) 2 = 3 ± 5 4028 f(2014)^2=\frac{3\pm\sqrt{5}}{4028} .

Next, we square root both sides and get f ( 2014 ) = ± 3 ± 5 4028 f(2014)=\pm\sqrt{\frac{3\pm\sqrt{5}}{4028}} . We can throw out the negative values of f ( 2014 ) f(2014) since they're not going to be the largest value of f ( 2014 ) f(2014) . We see that 3 5 4028 0.01377 \sqrt{\frac{3-\sqrt{5}}{4028}}\approx0.01377 and 3 + 5 4028 0.03605 \sqrt{\frac{3+\sqrt{5}}{4028}}\approx0.03605 , so we take the larger value, which is 3 + 5 4028 0.03605 \sqrt{\frac{3+\sqrt{5}}{4028}}\approx0.03605 , and let it be M M .

Lastly, we find 1000 M \lfloor1000M\rfloor , which is 1000 3 + 5 4028 36.05 = 36 \lfloor1000\sqrt{\frac{3+\sqrt{5}}{4028}}\rfloor \approx \lfloor36.05\rfloor = \boxed{36} .

Yea whoops I messed up when I was making teh question harder and instead made it easier. So Germain Identity is useless now.

Daniel Liu - 7 years ago

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yes...This is a direct biquadratic equation...

Vinay Sipani - 7 years ago

Yeah. I'm surprised that something so easy would be one of Daniel's problems! It's just a 1 variable quadratic, for gosh sake! :D

Finn Hulse - 7 years ago

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Tis' a pity

Milly Choochoo - 6 years, 12 months ago

whoops.. I find it difficult.. I'm such a lame :(

Figel Ilham - 6 years, 6 months ago

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