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(1/20+2/20+3/20+...+19/20)-(2/40+4/40+6/40+...+38/40)=

Răspuns :

(1/20+2/20+3/20+...+19/20)-(2/40+4/40+6/40+...+38/40)=
= 1/20(1 + 2 + 3+...+19) - 2/40(1+2+3+...+19) =
= (1+2+3+...+19) (1/20-2/40) = 0