2010年10月17日 星期日

Polynomial roots

An elementary problem seen from other site.

Let be a polynomial with odd integral coefficients,

show that it cannot have rational root.

5 則留言:

  1. You may use similar method of proving root 2 is not a rational number to solve this problem.

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  2. the aim is to prove the sq root of delta is irrational ?
    actually i can not solve this problem by the method contradiction...

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  3. Suppose there is a rational root x=p/q, where p, q are relatively prime integer.

    put x=p/q into the polynomial.

    Note: a, b and c are odd integral coeff.

    finally check the parity.

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  4. Suppose LHS can be factorized into (Ax+B)(Cx+D), where A, B, C & D are integers.
    Then since a & c are odd, all A, B, C & D must be odd.
    Expand to get the coeff. of x = AD+BC =b
    b is odd but AD+BC is even.
    Therefore, contradiction.

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