Saturday 30 January 2016

taylor expansion - Proof an inequality



I'm trying to prove that
$$ \frac{3-2\sqrt{1-15 m^2}}{1+12 m^2}\geq 1+3 m^2$$



I have obtained in a CAS software the Taylor expansion in $m=0$
enter image description here




One posibility to prove the inequality is showing coeficients in Taylor expansion are non-negative, by I don't find how.



Really I want only to obtain inequality. Some idea?



EDIT



$m$ must be between $0

Answer



$$ \frac{3-2\sqrt{1-15m^2}}{1+12m^2} \geq 1+3m^2 \iff $$
$$ 3-2\sqrt{1-15m^2} \geq (1+3m^2)(1+12m^2) \iff $$

$$ 2\sqrt{1-15m^2} \leq 3-(1+3m^2)(1+12m^2) \iff $$
$$ \sqrt{1-15m^2} \leq \frac{3-(1+3m^2)(1+12m^2)}{2} \iff $$
$$ \sqrt{1-15m^2} \leq \frac{2-15m^2-36m^4}{2} $$
Note that on the interval you're concerned about, the right hand side is always positive. Proof: it's obviously decreasing on $\left(0,\frac{1}{\sqrt{15}}\right)$, and is equal to $\frac{21}{50}$ at the right endpoint. Therefore squaring both sides is legal here with an $\iff$ statement.
$$ \sqrt{1-15m^2} \leq \frac{2-15m^2-36m^4}{2} \iff $$
$$ 1-15m^2 \leq \left(\frac{2-15m^2-36m^4}{2}\right)^2 \iff $$
$$ 1-15m^2 \leq 324m^8 + 270m^6 + \frac{81}{4}m^4 - 15m^2 + 1 \iff $$
$$ 0 \leq 324m^8 + 270m^6 + \frac{81}{4}m^4 $$



This last statement is clearly true.



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