U3 4 u9 256 maka u15

Un = a × r⁽ⁿ⁻¹⁾

U₃ = 4
a × r⁽³⁻¹⁾ = 4
a.r² = 4

U₉ = 256
a × r⁽⁹⁻¹⁾ = 256
a.r⁸ = 256

Mencari nilai "r"
U₉ / U₃ = 256 / 4
(a.r⁸) / (a.r²) = 64
r⁽⁸⁻²⁾ = 64
r⁶ = 2⁶ ............. aˣ = bˣ ⇒ a = ˣ√bˣ ⇒ a = b⁽ˣ/ˣ⁾ ⇒ a = b
r = 2

Mencari nilai "a"
a × r² = 4
a × (2²) = 4
a × 4 = 4
a = 4/4
a = 1

Mencari U₁₅
Un = a × r⁽ⁿ⁻¹⁾
U₁₅ = (1) × (2)¹⁴
U₁₅ = (2)¹⁴
U₁₅ = 16.384