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[tex]\it E(k)=2k,\ \ \ E(k+1)=2(k+1)=2x+2\\ \\ \\ E(k)-E(k+1)=2k-2k-2=-2\\ \\ \\ \underbrace{ E(1)-E(2)}_{-2}+ \underbrace{ E(3)-E(4)}_{-2}+\ ...\ + \underbrace{ E(2019)-E(2020)}_{-2}=\\ \\ \\ = \underbrace{ -2-2-2-\ ...\ -2}_{1010\ termeni}=-2\cdot1010=-2020[/tex]
