Question

a1=10
an=2-2a
n=6
r=?
S6=?

Answer 1

Răspuns:

Explicație pas cu pas:

n=6, a₁=10; aₙ=-20, deci a₆=-20,  dar a₆=a₁+5r, deci a₁+5r=a₆, 10+5r=-20,  5r=-20-10,  5r=-30,  r=-30:5=-6.

S₆=6·(a₁+a₆)/2=3·(10+(-20))=3·(-10)=-30.