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I know one thing: that I know nothing.

I know that I know nothing.

--Socratic paradox

I know that I know nothing.

--Socratic paradox

$$\cos x = \sum\limits_{n = 0}^\infty {\frac{{\left( { - 1} \right)^n x^{2n} }}{{\left( {2n} \right)!}}}$$