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DETERMINA: 6+11+16+21+......+506=

Răspuns :

6=1+5·111=1+5·216=1+5·3...............506= 1+5·101 ⇒ S = 1·101 + 5(1+2+3+.......+101)= 101+5·101·102/2 = 101+25755 = 25856