7log 2 = a, 5log 7 = b, maka 5log 40 jika dinyatakan dengan a dan b adalah ...

7log 2 = a
log 2/ log 7 = a
log 2 = a log 7

5log 7 = b
log 7/ log 5 = b
log 7 = b log 5

log 2 = a log 7
log 2 = ab log 5

5log40 = log 40/log 5 = log 5 . 2 . 2 . 2 / log 5 = log 5 + 3 . log 2 / log 5 = log 5 + 3 . ab log 5 / log 5 = log 5 (1 + 3ab) / log 5 = 1 + 3ab

⁷log2 = a
⁵log7 = b
b . a = ⁵log7 . ⁷log2
ab = ⁵log2

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⁵log40
= ⁵log(5 . 8)
= ⁵log5 + ⁵log2³
= 1 + 3 .⁵log2
= 1 + 3ab