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calculati a unde a=5+5^2+.....+5^10
Cat se poate de repede va rog.
Multumec anticipat!


Răspuns :

a=5+5^2+...+5^10/×5=>
5a= 5^2+...+5^10+5^11
5^2+5^3+...+5^10=a-5
=>5a=a-5+5^11=>a=(5^11-5)/4