How do you solve log_3x=log_9 7x-6?

1 Answer
Mar 21, 2016

x=7/531441

Explanation:

1. Start by moving all logs to the left side of the equation.

log_3(x)=log_9(7x)-6

log_3(x)-log_9(7x)=-6

2. Use the change of base formula, log_color(blue)n(color(red)m)=(log_color(purple)b(color(red)m))/(log_color(purple)b(color(blue)n)), to rewrite log_9(7x) with a base of 3.

log_3(x)-(log_3(7x))/(log_3(9))=-6

3. Use the log property, log_color(purple)b(color(purple)b^color(orange)x)=color(orange)x, to rewrite log_3(9).

log_3(x)-(log_3(7x))/(log_3(3^2))=-6

log_3(x)-(log_3(7x))/2=-6

4. Use the log property, log_color(purple)b(color(red)m^color(blue)n)=color(blue)n*log_color(purple)b(color(red)m), to rewrite (log_3(7x))/2.

log_3(x)-1/2(log_3(7x))=-6

log_3(x)-log_3((7x)^(1/2))=-6

5. Use the log property, log_color(purple)b(color(red)m/color(blue)n)=log_color(purple)b(color(red)m)-log_color(purple)b(color(blue)n) to simplify the left side of the equation.

log_3(x/((7x)^(1/2)))=-6

log_3(x^(1/2)/7^(1/2))=-6

6. Use the log property, log_color(purple)b(color(purple)b^color(orange)x)=color(orange)x, to rewrite the right side of the equation.

log_3(x^(1/2)/7^(1/2))=-log_3(3^6)

7. Use the log property, log_color(purple)b(color(red)m^color(blue)n)=color(blue)n*log_color(purple)b(color(red)m), to rewrite the right side of the equation.

log_3(x^(1/2)/7^(1/2))=log_3(3^-6)

8. Since the equation now follows a "log=log" situation, where the bases are the same on both sides, rewrite the equation without the "log" portion.

x^(1/2)/7^(1/2)=3^-6

9. Solve for x.

x^(1/2)/7^(1/2)=1/3^6

x^(1/2)/7^(1/2)=1/729

x^(1/2)=7^(1/2)/729

(x^(1/2))^2=(7^(1/2)/729)^2

color(green)(|bar(ul(color(white)(a/a)x=7/531441color(white)(a/a)|)))