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Friday, April 14, 2017

2017/010) Let a,\,b,\,c,\,d be real with condition that a+b\sqrt{2}+c\sqrt{3}+2d >= \sqrt{10(a^2+b^2+c^2+d^2)}

show that a^2+d^2=b^2+c^2

Proof:

by Cauchy-Schawartz in equality we have
(a^2+b^2+c^2+d^2)(1^2 + \sqrt2^2+ \sqrt3^2+2^2)>=(a+b\sqrt{2}+c\sqrt{3}+2d)^2
Or 10(a^2 + b^2 + c^2+ d^2)>=(a+b\sqrt{2}+c\sqrt{3}+2d)^2
Or \sqrt{10(a^2 + b^2+ c^2+d^2)}>=(a+b\sqrt{2}+c\sqrt{3}+2d)
from given condition and above we have
\sqrt{10(a^2 + b^2+c^2+d^2)}=(a+b\sqrt{2}+c\sqrt{3}+2d)^2
when
\frac{a}{1}= \frac{b}{\sqrt2}=\frac{c}{\sqrt3}= \frac{d}{2}= k (say)
So a = k, b^2= 2k^2,c^2=3k^2,d=2k and hence a^2+d^2=b^2+c^2

Sunday, April 2, 2017

2017/009}Prove that \sqrt{\frac{1\cdot 2}{3^2}}+\sqrt{\frac{2\cdot 3}{5^2}}+\sqrt{\frac{3\cdot 4}{7^2}}+\cdots+\sqrt{\frac{4032\cdot 4033}{8065^2}}\lt 2016

we have n^{th} term = \frac{\sqrt{n\cdot (n+1)}}{2n+1}
= \frac{\sqrt{n^2+n}}{2n+1}
= \frac{\sqrt{n^2+n+\frac{1}{4}-\frac{1}{4}}}{2n+1}
=  \frac{\sqrt{(n+\frac{1}{2})^2-\frac{1}{4}}}{2n+1}
< \frac{n+\frac{1}{2}}{2n+1}
< \frac{1}{2}
each term is < \frac{1}{2} and there are 4032 terms so sum is less than 2016