Răspuns:
S20=2420
Explicație pas cu pas:
a14=170
a14=a1+13r
=>
a1+13r=170 (1)
a3=16
a3=a1+2r
=>
a1+2r=16 (2)
(1)-(2)=> 11r=154
r=154/11
r=14
a3=a1+2r =>16=a1+2*14 =>a1= 16-28 => a1= -12
n=20 => a20= a1+19r => a20= -12+ 19*14 => a20= -12 + 266 => a20=254
Sn= [(a1+an)n] /2= [(a1+a20)n}]/2=[(-12+254)20]/2=(242*20)/2=4840/2=2420
