Răspuns:
Explicație pas cu pas:
u3=u2×q=u1×q²
u6=100=u5×q=u4×q²=u3×q³=4/5×q³=100 ⇒q³/5=5² q=∛125=5
u1×q²=4/5 u1×5²=4/5 u1=2/5√5=2√5/25
n=3
Sn=n(u1+u3)/2 S3=3(2√5/25+4/5)/2=6(√5+10)/25·2=3(√5+10)/25
u3n-1=u1×5^3n=2×5^(1/2)×5^3n=2×5^(3n+1/2)
