有關log既數唔係好識點做 [急]

1 令y = log_2 (x - 4)

原式變成y^2 - 2y - 3 = 0

(y - 3)(y + 1) = 0

y = 3 或 -1 (捨去)

log_2 (x - 4) = 3

x - 4 = 2^3 = 8

x = 12

2 log_5 (8x - 11) = 2 + log_1/5 (2x + 1)

log(8x - 11)/log5 = 2 + log(2x + 1)/log(1/5)

log(8x - 11)/log5 = 2 - log(2x + 1)/log5

log(8x - 11) = 2log5 - log(2x + 1)

log(8x - 11) = log25 - log(2x + 1)

log(8x - 11) = log[25/(2x + 1)]

8x - 11 = 25/(2x + 1)

(8x - 11)(2x + 1) - 25 = 0

8x^2 -7x - 18 = 0

(8x + 9)(x - 2) = 0

x = -9/8 (捨去) 或 2

1
quad equation
log2(x-4)=(2士((4+12)^0.5))/2
=3 or -1

when log2(x-4)=3
x-4=8
x=12

when log2(x-4)=-1
x-4=1/2
x=9/2

x=12 or 9/2


2
log5 (8x-11) = 2 + log1/5 (2x+1)
[log1/5(8x-11)]/[log1/5(5)]=2+log1/5 (2x+1)
-log1/5(8x-11)-log1/5(2x+1)=2
log1/5(8x-11)+log1/5(2x+1)=-2
(8x-11)*(2x+1)=25
16x^2-14x-36=0
x=2 or -9/8(rejected)