Jika 2 log 3 = a, dan 3 log 5 = b maka 15 log 20 adalah

15log20
= 3log20 / 3log15
= 3log(2 x 2 x 5) / 3log(5 x 3)
= (3log2 + 3log2 + 3log5) / (3log5 + 3log3)
= (1/a + 1/a + b) / (b + 1)
= (2/a + b ) / (b + 1)

= [(2 + ab) / a ] / (b + 1)
= (2 + ab) / (a (b + 1))