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Ronaldo
a fost răspuns

5+10+15+20+.......2015 

2 la puterea x - 1 = 1+2+2 la puterea a 2 ..... 2 la puterea a 10

Răspuns :

Deva
5(1+2+3+...+403)=5(403x404/2)=5(403x202)=5x403x202=
407030
2lax-1=1+2+2la2+...2la10
2lax=2+2(1+2=...+2la9)
2lax=2+2x(512x513/2)
2lax=2+512x513
2lax=2+262656=262658


Bunicaluiandrei
S = 5(1+2+3+.......+403) = 5·403·404/2 = 5·403·202=407030
2^x - 1 =1+2+2² +2³ +........+2^10
daca S = 1+2+2² +.........+2^10  si 2S=2+2²+.........+2^11 ⇒ 2S-S =S = 2^11 - 1
2^x - 1 = 2^11 - 1 ⇒ 2^x - 2^11 ⇒ x=11