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Prove that for any odd integer m, the number (m4 + 4m2 + 11)/16 is an integer.

Mathematics

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Step-by-step solution

19 steps
  1. Let mm be an odd integer.

    We can express any odd integer mm in the form m=2k+1m = 2k+1 for some integer kk.

  2. Find expressions for m2m^2 and m4m^4 in terms of kk.

    m2=(2k+1)2=4k2+4k+1=4k(k+1)+1m^2 = (2k+1)^2 = 4k^2 + 4k + 1 = 4k(k+1) + 1

    Since kk and k+1k+1 are consecutive integers, one of them must be even. Therefore, k(k+1)k(k+1) is always an even integer. Let k(k+1)=2jk(k+1) = 2j for some integer jj.
    Then, m2=4(2j)+1=8j+1m^2 = 4(2j) + 1 = 8j+1.
    Now, find m4m^4:

    m4=(m2)2=(8j+1)2=(8j)2+2(8j)(1)+12=64j2+16j+1m^4 = (m^2)^2 = (8j+1)^2 = (8j)^2 + 2(8j)(1) + 1^2 = 64j^2 + 16j + 1
  3. Substitute these expressions into the given number.

    The number is m4+4m2+1116\frac{m^4 + 4m^2 + 11}{16}.
    Substitute m4=64j2+16j+1m^4 = 64j^2 + 16j + 1 and m2=8j+1m^2 = 8j+1:

    (64j2+16j+1)+4(8j+1)+1116\frac{(64j^2 + 16j + 1) + 4(8j+1) + 11}{16} 64j2+16j+1+32j+4+1116\frac{64j^2 + 16j + 1 + 32j + 4 + 11}{16} 64j2+(16j+32j)+(1+4+11)16\frac{64j^2 + (16j + 32j) + (1 + 4 + 11)}{16} 64j2+48j+1616\frac{64j^2 + 48j + 16}{16}
  4. Simplify the expression.

    Factor out 16 from the numerator:

    16(4j2+3j+1)16\frac{16(4j^2 + 3j + 1)}{16} 4j2+3j+14j^2 + 3j + 1

    Since jj is an integer, 4j2+3j+14j^2 + 3j + 1 is also an integer.
    Therefore, the given number is an integer.

    The number is an integer.\boxed{\text{The number is an integer.}}
  5. Combine the square roots using the property x×y=xy\sqrt{x} \times \sqrt{y} = \sqrt{xy}.

    ba×cb×ac=ba×cb×ac\sqrt{\frac{b}{a}} \times \sqrt{\frac{c}{b}} \times \sqrt{\frac{a}{c}} = \sqrt{\frac{b}{a} \times \frac{c}{b} \times \frac{a}{c}}
  6. Multiply the fractions inside the square root.

    b⋅c⋅aa⋅b⋅c\sqrt{\frac{b \cdot c \cdot a}{a \cdot b \cdot c}}
  7. Cancel out common terms in the numerator and denominator.

    abcabc=1\sqrt{\frac{abc}{abc}} = \sqrt{1}
  8. Evaluate the square root.

    1=1\sqrt{1} = 1

    Thus, ba×cb×ac=1\sqrt{\frac{b}{a}} \times \sqrt{\frac{c}{b}} \times \sqrt{\frac{a}{c}} = 1.

    ba×cb×ac=1\boxed{\sqrt{\frac{b}{a}} \times \sqrt{\frac{c}{b}} \times \sqrt{\frac{a}{c}} = 1}
  9. The problem states that n=32\sqrt{n} = \frac{3}{2}.

  10. We need to find the value of −n-\sqrt{n}.

    Substitute the given value of n\sqrt{n} into the expression:

    −n=−(32)-\sqrt{n} = -\left(\frac{3}{2}\right) −n=−32-\sqrt{n} = -\frac{3}{2} −32\boxed{-\frac{3}{2}}
  11. Start with the known inequality that the square of any real number is non-negative.

    For any real numbers aa and bb, (a−b)2≥0(a-b)^2 \ge 0.

  12. Expand the inequality.

    a2−2ab+b2≥0a^2 - 2ab + b^2 \ge 0
  13. Since aa and bb are positive real numbers, their product abab is also positive.

    We can divide the entire inequality by abab without changing the direction of the inequality sign.

    a2−2ab+b2ab≥0ab\frac{a^2 - 2ab + b^2}{ab} \ge \frac{0}{ab} a2ab−2abab+b2ab≥0\frac{a^2}{ab} - \frac{2ab}{ab} + \frac{b^2}{ab} \ge 0
  14. Simplify the terms.

    ab−2+ba≥0\frac{a}{b} - 2 + \frac{b}{a} \ge 0
  15. Add 2 to both sides of the inequality.

    ab+ba≥2\frac{a}{b} + \frac{b}{a} \ge 2

    This proves the statement.

    ab+ba≥2\boxed{\frac{a}{b} + \frac{b}{a} \ge 2}
  16. Start with the known inequality that the square of any real number is non-negative.

    Since xx is a real number and x≠0x \ne 0, x−1xx - \frac{1}{x} is a real number.
    Therefore, (x−1x)2≥0\left(x - \frac{1}{x}\right)^2 \ge 0.

  17. Expand the square.

    (x−1x)2=x2−2⋅x⋅1x+(1x)2\left(x - \frac{1}{x}\right)^2 = x^2 - 2 \cdot x \cdot \frac{1}{x} + \left(\frac{1}{x}\right)^2 x2−2+1x2x^2 - 2 + \frac{1}{x^2}
  18. Substitute the expanded form back into the inequality.

    x2−2+1x2≥0x^2 - 2 + \frac{1}{x^2} \ge 0
  19. Add 2 to both sides of the inequality.

    x2+1x2≥2x^2 + \frac{1}{x^2} \ge 2

    This proves the statement.

    x2+1x2≥2\boxed{x^2 + \frac{1}{x^2} \ge 2}
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