Question

Aflati suma unei progresii aritmetice cu ratia 2 si a1-an=-20

Answer 1

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Answer 2

r=2;  a1-an=-20

an=a1+(n-1)r=a1+2(n-1)

an=a1+2n-2

a1-an=a1-a1-2n+2=-2n+2=-20

n=(-20-2)/(-2)

n=11

a11=a1+20

S11=11(a1+a1+20)/2

S11=11·2(a1+10)/2

S11=11(a1+10)