A different type of algebra problem 2

Algebra Level 5

n = 1 5 x n 2 = n = 1 4 x ( n ) n \large \displaystyle\sum _{ n=1 }^{ 5 }{ { x }^{ { n }^{ 2 } } } =\displaystyle\sum _{ n=1 }^{ 4 }{ { x }^{ { (-n) }^{ n } } }

How many real solutions does the above equation have?

Enter 1729 if you think that there are an infinite number of solutions

Bonus : Find all these solutions.


The answer is 3.

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1 solution

It's an really interesting problem & I'm just posting my approach of solution as I'm unsure about it.

Expanding we get ,

x + x 4 + x 9 + x 16 + x 25 = 1 x + x 4 + 1 x 27 + x 256 + 1 x 3125 x+x^4+x^9+x^{16}+x^{25}=\frac{1}{x} + x^4 + \frac{1}{x^{27}} + x^{256} + \frac{1}{x^{3125}}

So Just transforming it into a equation by multiplying x 3125 x^{3125} to both sides we have ,

f ( x ) = x 3381 x 3150 x 3141 x 3134 x 3126 + x 3124 + x 3098 + 1 = 0 f(x)=x^{3381} - x^{3150} - x^{3141} - x^{3134} - x^{3126} + x^{3124} + x^{3098} + 1=0

f ( x ) = x 3381 x 3150 + x 3141 x 3134 x 3126 + x 3124 + x 3098 + 1 = 0 f(-x)=-x^{3381} - x^{3150} + x^{3141} - x^{3134} - x^{3126} + x^{3124} + x^{3098} + 1=0

Applying Descarte's Rule of sign to the polynomial we have there are two sign changes in f ( x ) f(x) i.e.

+ x 3381 \color{#D61F06}+x^{3381} to x 3150 \color{#3D99F6}-x^{3150} & x 3126 \color{#3D99F6}-x^{3126} to + x 3124 \color{#D61F06}+x^{3124}

So it has atmost 2 positive real roots.

Again we have three sign changes in f(-x) i.e. ,

x 3150 \color{#3D99F6}-x^{3150} to + x 3141 \color{#D61F06}+x^{3141} & + x 3141 \color{#D61F06}+x^{3141} to x 3134 \color{#3D99F6}-x^{3134} & x 3126 \color{#3D99F6}-x^{3126} to + x 3124 \color{#D61F06}+x^{3124} .

So it has atmost 3 negetive real roots .

Now in f ( x ) f(-x) we have Product of roots = a n a 0 = -\frac{a_n}{a_0} = 1 1 = 1 -\frac{1}{-1}=1 So there are 2 or 0 negetive roots .

In the original equation we have product of roots = 1 -1 so we must have a negative root. The rest of the roots are complex conjugates whose product is always positive as z . z ˉ = ( z ) 2 z.\bar{z}=(|z|)^2

So we have 1 + 2 = 3 \boxed{1}+\boxed{2}=\boxed{3} real solutions.

Aditya, RHS only have 4 terms...

Guillermo Templado - 4 years, 8 months ago

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