Question

Solve log4 (y – 9) + log4 3 = log4 81.

Answer 1

y = 36

Explanation:

log4 (y – 9) + log4 3 = log4 81.

We know from the property of logs that if the bases are the same, when we subtract logs, we can divide what is inside

loga (b) + loga (b) = loga (b*c)

Lets simplify the expression log4( (y-9)*3) = log4 (81)

Since the bases are the same, what is inside the parentheses on the left hand side must be equal to what is inside the parentheses o the right hand side

(y-9)*3 = 81

Divide each side by 3

(y-9)*3 /3= 81/3

y-9 = 27

Add 9 to each side

y-9+9 = 27+9

y = 36

Answer 2

`\text{Domain}\\\\y-9 > 0\to y>9\\\\------------------------\\\\\log_4(y-9)+\log_43=\log_481\qquad\text{use}\ \log x+\log y=\log(xy)\\\\\log_4[3(y-9)]=\log_481\qquad\text{use distributive property}\\\\\log_4(3y-27)=\log_481\iff3y-27=81\qquad\text{add 27 to both sides}\\\\3y=108\qquad\text{divide both sides by 3}\\\\\boxed{y=36}\in D\\\\Answer:\ \boxed{y=36}`