Given the equation
4 x 3 + 7 y 3 = 2 0 1 7
How many pairs of positive integers ( x , y ) that satisfy the equation?
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Since RHS is 3 modulo 4 so must be LHS.So substituting y=4k-1 we transform the equation into x^3+112k^3+21k-84k^2=506.Now since RHS is 2 modulo 7 so must be LHS.But in lhs except x^3 every term is divisible by 7.So x^3 is 2 modulo 7 which is not possible since cubic residues of 7 are 0,1,and 6.
Well I don't know if its correct or not, but I did it this way:- 4 x 3 + 7 y 3 = 2 0 1 7 ( 4 1 / 3 x ) 3 + ( 7 1 / 3 y ) 3 = 2 0 1 7
Now as 2017 is prime, the x + y term in x 3 + y 3 must be equal to 1 . But ( 4 1 / 3 x + 7 1 / 3 y ) can never be equal to 1 for non negative values of x and y. Thus there are 0 solutions
I really do not mean to be rude, but this is hilarious.
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We know that 2 0 1 7 ≡ 1 ( m o d 7 ) . So, we can conclude that 4 x 3 + 7 y 3 ≡ 1 ( m o d 7 ) 4 x 3 ≡ 1 ( m o d 7 ) x 3 ≡ 2 ( m o d 7 ) For x ≡ 0 , 1 , 2 , 3 , 4 , 5 , 6 ( m o d 7 ) . There will be x 3 ≡ 0 , 1 , 6 ( m o d 7 ) . So, there are no solution in postive integers for x . And we can conclude that there are no pairs of positive integers ( x , y )