Jika 2log3=a dan 3log5=b, maka 15log20 adalah...

²log 3 = a
³log 5 = b
²log5 = ²log3 . ³log5 = a.b

¹⁵log20
= ²log20 / ²log15
= (²log(2²x5)) / (²log(3x5))
= (2.²log2 + ²log5) / (²log3 + ²log5)
= (2.1 + a.b) / (a + a.b)
= (2 + ab) / (a + ab)