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May 17, 2014 at 13:24 vote accept pedro
May 17, 2014 at 13:20 comment added pedro I think that once we have that $\cos\theta+\frac{|v|^2}2\ge 1$, then can conclude easily by using that $$|v|^2 \leq |x|^2 + (1-\sqrt{1-|x|^2})^2,$$ for $|x|\leq1$, which follows by writing the touching spheres at $O$. Indeed we find that $\cos\theta \geq \sqrt{1-|x|^2}$ and we conclude. Thank you again for your answer!
May 16, 2014 at 17:57 history answered fedja CC BY-SA 3.0