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(an>=1) - progresie artimetica
a2+a9=10
s10=?


Răspuns :

{a2=a1+r
{a9=a1+8r  Le  adui
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a2+a9=a1+a1+9r=10  Dar  a1+9r=a10  =>
a1+a10=1
S10=(a1+a10)*1/2=10*10/2=50