唔識做功課72

log 3 = a
log 2 = b

(a)
log 240
= log(2⁴ × 3 × 5)
= log(2³ × 3 × 10)
= 3 log2 + log3 + log10
= 3b + a + 1
= a + 3b + 1

(b)
log₃√45
= log√45 / log 3
= log√(3² × 5) / log 3
= log√(3² × 10 ÷ 2) / log 3
= log[(3² × 10 ÷ 2)^(1/2)] / log 3
= (1/2)log[(3² × 10 ÷ 2)] / log 3
= (1/2)(2 log3 + log10 - log2) / log 3
= (2a + 1 - b) / (2a)
= (2a - b + 1)/(2a)

2015-06-11 16:58:02 補充：
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我做到忘記咗√ 係等於２分１次方,唔該晒!!!