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Calculati 5+9+13+...+401. Repede, va rog.

Răspuns :

= 5 + (5+1*4) + (5+2*4) + (5+3*4) + ... + (5+99*4) =
= 100*5 + 4*(1+2+3+...+99)=
= 500 + 4* (99*100:2) = 500 + 4*99*50 = ...