兩題log數唔識做

a) ln (x2 + 5x) = 2 ln (x + 1)

ln (x2 + 5x) = ln (x + 1)2

x2 + 5x = (x + 1)2

x2 + 5x = x2 + 2x + 1

3x = 1

x = 1/3

b) log (7x - 12) = 2 log x

log (7x - 12) = log x2

7x - 12 = x2

x2 - 7x + 12 = 0

(x - 4)(x - 3) = 0

x = 4 or 3

利用 2ln(x+1) = ln(x+1)^2
利用 2logx = logx^2
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