U4+U6=10 jika rasio 1/2 maka U8 adalah

U4 = a r³
      = a (1/2)³
U6 = a r⁵
     = a (1/2)⁵
     = a (1/2)³ (1/2)²

U4 + U6 = 10
a (1/2)³ + a (1/2)³ (1/2)²  = 10
a (1/2)³ (1 + 1/4) = 10
a (1/8) (5/4) = 10
         a(1/8) = 10 x 4/5
               a  = 8 x 8
                   = 64
U8 = a r⁷
     = 64 (1/2)⁷
     = 64 (1/128)
     = 0,5