Răspuns :
pai daca... s10=100(10*10=100) si s30=900(30*30=900) atunci s50=2500(50*50=2500)
s10=s1+9r=100
s30=s1+29r=900
=>
r=40 => s1=-260 =>
s50=s1+49r=-260 + 49 × 40 = 1700
s30=s1+29r=900
=>
r=40 => s1=-260 =>
s50=s1+49r=-260 + 49 × 40 = 1700