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If a2+b2=7ab, prove that log((a+b)/3)=1/2(log a + log b)

Mathematics

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Problem:
If a2+b2=7aba^2 + b^2 = 7ab, prove that log⁡(a+b3)=12(log⁡a+log⁡b)\log \left(\frac{a+b}{3}\right) = \frac{1}{2}(\log a + \log b).

  1. Start with the given equation and add $2ab$ to both sides to form a perfect square.

    a2+b2=7aba^2 + b^2 = 7ab a2+b2+2ab=7ab+2aba^2 + b^2 + 2ab = 7ab + 2ab (a+b)2=9ab(a+b)^2 = 9ab
  2. Take the square root of both sides.

    We assume a,b>0a, b > 0 for the logarithms to be defined, which implies a+b>0a+b > 0.

    (a+b)2=9ab\sqrt{(a+b)^2} = \sqrt{9ab} a+b=3aba+b = 3\sqrt{ab}
  3. Divide both sides by 3.

    a+b3=ab\frac{a+b}{3} = \sqrt{ab}
  4. Take the logarithm of both sides.

    log⁡(a+b3)=log⁡(ab)\log \left(\frac{a+b}{3}\right) = \log (\sqrt{ab})
  5. Apply the logarithm property log⁡(xn)=nlog⁡x\log(x^n) = n \log x.

    log⁡(a+b3)=log⁡((ab)1/2)\log \left(\frac{a+b}{3}\right) = \log ((ab)^{1/2}) log⁡(a+b3)=12log⁡(ab)\log \left(\frac{a+b}{3}\right) = \frac{1}{2} \log (ab)
  6. Apply the logarithm property log⁡(xy)=log⁡x+log⁡y\log(xy) = \log x + \log y.

    log⁡(a+b3)=12(log⁡a+log⁡b)\log \left(\frac{a+b}{3}\right) = \frac{1}{2} (\log a + \log b)

    This completes the proof.

    Proven: log⁡(a+b3)=12(log⁡a+log⁡b)\boxed{\text{Proven: } \log \left(\frac{a+b}{3}\right) = \frac{1}{2}(\log a + \log b)}
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