А₂₆=a₁+(n-1)d
а₂₆=5+(26-1)*(-2)= 5+(-50)= 5-50=-45
(х+6у)^2-(6у-х)^2=х^2+12ху+36у^2-(36у^2-12ху+х^2=х^2+12ху+36у^2-36у^2+12ху-х^2=24ху ( остальное сократилось).
Левую часть неравенства представим в виде
![(a-2)^2+2>0](https://tex.z-dn.net/?f=%28a-2%29%5E2%2B2%3E0)
. Откуда видим что левая часть принимает положительные значения, значит для всех действительных а выполняется исходное неравенство
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1) Пусть n=2
![1+ \frac{1}{ \sqrt{2} } \ \textgreater \ \sqrt{2} \\ \\ \sqrt{2}* (1+ \frac{1}{ \sqrt{2} } ) \ \textgreater \ \sqrt{2} *\sqrt{2} \\ \\ \sqrt{2} +1\ \textgreater \ 2 \\ \\ \sqrt{2} \ \textgreater \ 1 \\ \\](https://tex.z-dn.net/?f=1%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7B2%7D+%7D++%5C+%5Ctextgreater+%5C++%5Csqrt%7B2%7D++%5C%5C++%5C%5C++%5Csqrt%7B2%7D%2A+%281%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7B2%7D+%7D+%29+%5C+%5Ctextgreater+%5C+++%5Csqrt%7B2%7D+%2A%5Csqrt%7B2%7D++%5C%5C++%5C%5C++%5Csqrt%7B2%7D+%2B1%5C+%5Ctextgreater+%5C+2+%5C%5C++%5C%5C+%5Csqrt%7B2%7D+%5C+%5Ctextgreater+%5C+1+%5C%5C++%5C%5C)
верно
2)Пусть верно при n=k
![1+ \frac{1}{ \sqrt{2} } + \frac{1}{ \sqrt{3} } +...+ \frac{1}{ \sqrt{k} } \ \textgreater \ \sqrt{k} \\ \\](https://tex.z-dn.net/?f=1%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7B2%7D+%7D+%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7B3%7D+%7D+%2B...%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%7D+%7D+%5C+%5Ctextgreater+%5C++%5Csqrt%7Bk%7D++%5C%5C++%5C%5C+)
3)докажем, что верно при n=k+1
![1+ \frac{1}{ \sqrt{2} } + \frac{1}{ \sqrt{3} } +...+ \frac{1}{ \sqrt{k} } + \frac{1}{ \sqrt{k+1} } \ \textgreater \ \sqrt{k}+ \frac{1}{ \sqrt{k+1} } \\ \\](https://tex.z-dn.net/?f=1%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7B2%7D+%7D+%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7B3%7D+%7D+%2B...%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%7D+%7D+%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%2B1%7D+%7D+%5C+%5Ctextgreater+%5C+%5Csqrt%7Bk%7D%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%2B1%7D+%7D++%5C%5C++%5C%5C+)
![\frac{1}{ \sqrt{k+1} } -](https://tex.z-dn.net/?f=%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%2B1%7D+%7D+-+)
положительное число
![\sqrt{k} + \frac{1}{ \sqrt{k+1} } \ \textgreater \ \sqrt{k+1} \\ \\ \sqrt{k+1}*( \sqrt{k} + \frac{1}{ \sqrt{k+1} } )\ \textgreater \ \sqrt{k+1} * \sqrt{k+1} \\ \\ \sqrt{k(k+1)} +1\ \textgreater \ k+1 \\ \\ \sqrt{k^2+k} \ \textgreater \ \sqrt{k^2} ;k \geq 2 \\ \\](https://tex.z-dn.net/?f=+%5Csqrt%7Bk%7D+%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%2B1%7D+%7D+%5C+%5Ctextgreater+%5C++%5Csqrt%7Bk%2B1%7D++%5C%5C++%5C%5C+%5Csqrt%7Bk%2B1%7D%2A%28+%5Csqrt%7Bk%7D+%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%2B1%7D+%7D+%29%5C+%5Ctextgreater+%5C++%5Csqrt%7Bk%2B1%7D+%2A+%5Csqrt%7Bk%2B1%7D++%5C%5C++%5C%5C++%5Csqrt%7Bk%28k%2B1%29%7D+%2B1%5C+%5Ctextgreater+%5C+k%2B1+%5C%5C++%5C%5C++%5Csqrt%7Bk%5E2%2Bk%7D+%5C+%5Ctextgreater+%5C++%5Csqrt%7Bk%5E2%7D+%3Bk+%5Cgeq+2+%5C%5C++%5C%5C+)
верно
⇒
![1+ \frac{1}{ \sqrt{2} } + \frac{1}{ \sqrt{3} } +...+ \frac{1}{ \sqrt{k} } + \frac{1}{ \sqrt{k+1} } \ \textgreater \ \sqrt{k+1} } \\ \\](https://tex.z-dn.net/?f=1%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7B2%7D+%7D+%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7B3%7D+%7D+%2B...%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%7D+%7D+%2B+%5Cfrac%7B1%7D%7B+%5Csqrt%7Bk%2B1%7D+%7D+%5C+%5Ctextgreater+%5C+%5Csqrt%7Bk%2B1%7D+%7D+%5C%5C+%5C%5C+)
ч.т.д.