Răspunsuri

2014-09-16T16:18:35+03:00
A₆+a₉+a₁₂+a₁₅=20 => a₁+5r+a₁+8r+a₁+11r+a₁+14r=20.
4a₁+38r=20 | :2
2a₁+19r=10

S₂₀= \frac{(a _{1} +a _{20})*20 }{2} =(a₁+a₁+19r)10=(2a₁+19r)*10.
 dar 2a₁+19r=10  => S₂₀=10*10=100.