??

Suku pertama = a = 2 Suku keempat = U4 = 128 U4 = a x r³ 128 = 2 x r³ r³ = 128/2 r³ = 64 r = ³√64 r = 4 S4 = a(r⁴-1)/(r-1) = 2 x (4⁴-1)/(4-1) = 2 x (256-1)/3 = 2x255/3 = 510/3 = 170 Panjang tali semula = 170cm