Logarithmic functions > Logarithmic equations
12345Logarithmic equations

Solutions to the exercises

Exercise 1
a

D f = -4 , , B ( f ) = , vertical asymptote x = -4 .

b

f ( x ) = 0 gives you log ( x + 4 ) = 1 3 and therefore x + 4 = 10 1 3 , so x = 10 3 - 4 .
Graph: x > 10 3 -4 .

Exercise 2
a

D g = 1 , , B g = , vertical asymptote x = 1 .

b

g ( x ) = - 14 gives you 1 3 log ( x - 1 ) = -2 and therefore x - 1 = ( 1 3 ) -2 = 9 so x = 10 .
Graph: 1 < x 10 .

Exercise 3
a

3 log ( x ) = 3 log ( 5 2 ) , so x = 5 2 = 25 .

b

1 3 log ( x ) = 1 3 log ( 5 2 ) , so x = 10 .

c

2 log ( x ) = 5 , so x = 2 5 = 32 .

d

5 log ( x ) = 5 log ( 5 3 ) + 5 log ( 3 4 ) = 5 log ( 5 3 3 4 ) , so x = 125 81 = 10125 .

e

x = 5 ( 2 - x ) gives you x = 10 6 = 5 3 .

f

5 log ( x ) = 5 log ( 5 3 ) + 5 log ( x 4 ) = 5 log ( 5 3 x 4 ) , therefore x = 125 x 4 , and so x ( 125 x 3 - 1 ) = 0 such that x = 0 x = 0.2 .
Since x = 0 is not allowed, the answer is x = 0.2 .

Exercise 4
a

D f = 0 , , B f = , vertical asymptote x = 0 .
D g = , 4 , B g = , vertical asymptote x = 4 .

b

log ( x ) = -1 + log ( 4 - x ) gives you log ( x ) = log ( 0.1 ( 4 - x ) ) .
This means: x = 0.4 - 0.1 x and therefore x = 0.4 1.1 = 4 11 .

c

0 < x 4 11

d

4 11 < x < 4

Exercise 5
a

q = 5 - 3 15 - p

b

q = 200 10 ( p - 600 ) 15

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