Isn't 100000 such a huge number (Part-8)?

Algebra Level 5

a 2 ( a 99998 + 1 ) + b 2 ( b 99998 + 1 ) + c 2 ( c 99998 + 1 ) = 2 ( a 50000 b + b 50000 c + c 50000 a ) \color{#69047E}{a^{2}(a^{99998}+1)+b^{2}(b^{99998}+1)+c^{2}(c^{99998}+1)=2(a^{50000}b+b^{50000}c+c^{50000}a})

How many ordered triples ( a , b , c ) \color{#20A900}{(a,b,c)} are there where a , b a,b and c c are complex numbers and a 50000 b , b 50000 c a^{50000}-b,b^{50000}-c and c 50000 a c^{50000}-a are rational numbers satisfying the equation above?

The answer can be expressed in the form x × 1 0 10 x\times 10^{10} .

Find the value of x x .

This problem is original.


The answer is 12500.

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1 solution

Yash Singhal
Jan 28, 2015

Expanding the brackets and transferring all the terms to one side, we get:

a 100000 + a 2 + b 100000 + b 2 + c 100000 + c 2 2 a 50000 b 2 b 50000 c 2 c 50000 a = 0 a^{100000}+a^{2}+b^{100000}+b^{2}+c^{100000}+c^{2}-2a^{50000}b-2b^{50000}c-2c^{50000}a=0

Rearranging the terms, we get:

( a 50000 b ) 2 + ( b 50000 c ) 2 + ( c 50000 a ) 2 = 0 (a^{50000}-b)^{2}+(b^{50000}-c)^{2}+(c^{50000}-a)^{2}=0 .

We can deduce from here that each and every term of the above equation is equal to zero as they are rational.

So, a 50000 = b , b 50000 = c a^{50000}=b, b^{50000}=c and c 50000 = a c^{50000}=a .

By substitution, we get:

( ( a 50000 ) 50000 ) 50000 ) = a ((a^{50000})^{50000})^{50000})=a .

a 125000000000000 = a a^{125000000000000}=a .

Clearly, a = 0 a=0 is a solution to this equation.

Now consider the case when a 0 a≠0 :

We can divide both sides by a a to get a 124999999999999 = 1 a^{124999999999999}=1 .

Now, we get 124999999999999 124999999999999 complex roots of unity and 1 1 solution when a = 0 a=0 .

So, in all there are 125000000000000 125000000000000 ordered triplets which can be represented as 12500 × 1 0 10 12500\times 10^{10} .

Hence, our final answer is 12500. \color{#D61F06}{\huge{12500.}}

Why should rational numbers be equal to 0? Please explain, thanks

Marvel Wijaya - 6 years, 4 months ago

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Square of a Rational Number is either 0 0 or a positive value. Since their sum is equal to 0 0 , they all must be 0 0 .

Yash Singhal - 6 years, 4 months ago

you got a^124999999999999=1 hence one root of a=1 must be neglected so there are 124999999999998 complex roots

Aakash Khandelwal - 6 years ago

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a = 1 a=1 will not be neglected because 1 1 is also a complex number.

Yash Singhal - 6 years ago

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