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Algebra Difficulty 6.8 National olympiad Prove it

If a>0,b>0,p>1,q>1a>0, b>0, p>1, q>1 and 1p+1q=1\frac{1}{p}+\frac{1}{q}=1 then ab1pap+1qbqa b \leqslant \frac{1}{p} a^{p}+\frac{1}{q} b^{q}

Solution

Prove by Lemma 1.x>0xaαx1α(011 . x>0 \quad x^{a}-\alpha x \leqslant 1-\alpha(01
and 1p+1q=1p+p1p=1\frac{1}{\mathrm{p}}+\frac{1}{\mathrm{q}}=\frac{1}{\mathrm{p}}+\frac{\mathrm{p}-1}{\mathrm{p}}=1
Secondly, for xaαx+1α(x>0,00,b>0,apbq>0)x^{a} \leqslant \alpha x+1-\alpha \quad(x>0,00, b>0, \frac{a^{p}}{b^{q}}>0) we get (apbq)1p1papbq+1q\left(\frac{a^{p}}{b^{q}}\right)^{\frac{1}{p}} \leqslant \frac{1}{p} \frac{a^{p}}{b^{q}}+\frac{1}{q}
that is
a(b)pq1papbq+1q\frac{a}{(b)_{p}^{q}} \leqslant \frac{1}{p} \frac{a^{p}}{b^{q}}+\frac{1}{q}

Now calculate bbpqp=b \cdot b_{p}^{\frac{q}{p}}= ?
By bbpq=b1+qp=bp+gpb \cdot b_{p}^{q}=b^{1+\frac{q}{p}}=b^{\frac{p+g}{p}}
and by 1p+1q=1p+q=pq\quad \frac{1}{\mathrm{p}}+\frac{1}{\mathrm{q}}=1 \quad \therefore \mathrm{p}+\mathrm{q}=\mathrm{pq}
Therefore bbpq=bp+qq=bppqp=bqb \cdot b_{p}^{q}=b^{\frac{p+q}{q}}=b_{p}^{\frac{p q}{p}}=b^{q}
Hence bbpg=bqb \cdot b_{p}^{g}=b^{q}
(2) Multiply both sides
bbpg=bqb \cdot b_{p}^{g}=b^{q}

We get bbpqp=bqabpq[1papbq+1q]b \cdot b_{p}^{\frac{q}{p}}=b^{q} \quad \frac{a}{b_{p}^{q}} \leqslant\left[\frac{1}{p} \frac{a^{p}}{b^{q}}+\frac{1}{q}\right]
That is ab1pap+1qbqa b \leqslant \frac{1}{p} a^{p}+\frac{1}{q} b^{q}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.