Math Problem Statement

((2log3(x))-4)/log3(x/9)=7log3(x)-(1/logx(3))^2-8

Solution

We are given the equation:

2log3(x)4log3(x9)=7log3(x)(1logx(3))28\frac{2 \log_3(x) - 4}{\log_3\left(\frac{x}{9}\right)} = 7 \log_3(x) - \left(\frac{1}{\log_x(3)}\right)^2 - 8

Step 1: Simplify both sides.

Left-hand side (LHS):

Using the property logb(mn)=logb(m)logb(n)\log_b\left(\frac{m}{n}\right) = \log_b(m) - \log_b(n):

log3(x9)=log3(x)log3(9)\log_3\left(\frac{x}{9}\right) = \log_3(x) - \log_3(9) Since log3(9)=2\log_3(9) = 2, we get: log3(x9)=log3(x)2\log_3\left(\frac{x}{9}\right) = \log_3(x) - 2 Thus, the LHS becomes: 2log3(x)4log3(x)2\frac{2 \log_3(x) - 4}{\log_3(x) - 2}

Right-hand side (RHS):

Next, simplify the RHS. First, simplify 1logx(3)\frac{1}{\log_x(3)}. Using the change of base formula, we know that: logx(3)=1log3(x)\log_x(3) = \frac{1}{\log_3(x)} Thus: 1logx(3)=log3(x)\frac{1}{\log_x(3)} = \log_3(x) Therefore: (1logx(3))2=(log3(x))2\left(\frac{1}{\log_x(3)}\right)^2 = \left(\log_3(x)\right)^2 The RHS becomes: 7log3(x)(log3(x))287 \log_3(x) - \left(\log_3(x)\right)^2 - 8

Step 2: Rewrite the equation.

Now, the equation is:

2log3(x)4log3(x)2=7log3(x)(log3(x))28\frac{2 \log_3(x) - 4}{\log_3(x) - 2} = 7 \log_3(x) - \left(\log_3(x)\right)^2 - 8

Step 3: Substitute y=log3(x)y = \log_3(x).

Let y=log3(x)y = \log_3(x), then the equation becomes:

2y4y2=7yy28\frac{2y - 4}{y - 2} = 7y - y^2 - 8

Step 4: Solve the equation.

Multiply both sides by y2y - 2 (assuming y2y \neq 2):

2y4=(7yy28)(y2)2y - 4 = (7y - y^2 - 8)(y - 2)

Expand the right-hand side:

2y4=(7y214yy3+2y28y+16)2y - 4 = (7y^2 - 14y - y^3 + 2y^2 - 8y + 16) 2y4=y3+9y222y+162y - 4 = -y^3 + 9y^2 - 22y + 16

Bring all terms to one side:

0=y3+9y224y+200 = -y^3 + 9y^2 - 24y + 20 y39y2+24y20=0y^3 - 9y^2 + 24y - 20 = 0

Step 5: Solve the cubic equation.

Now, we need to solve y39y2+24y20=0y^3 - 9y^2 + 24y - 20 = 0. We can attempt to factor this or use the Rational Root Theorem to find possible roots.

The possible rational roots are ±1,±2,±4,±5,±10,±20\pm 1, \pm 2, \pm 4, \pm 5, \pm 10, \pm 20.

Testing y=2y = 2:

239(22)+24(2)20=836+4820=02^3 - 9(2^2) + 24(2) - 20 = 8 - 36 + 48 - 20 = 0

Thus, y=2y = 2 is a solution.

Step 6: Factor the cubic polynomial.

Since y=2y = 2 is a root, we can factor y39y2+24y20y^3 - 9y^2 + 24y - 20 as (y2)(y27y+10)(y - 2)(y^2 - 7y + 10).

Now, solve y27y+10=0y^2 - 7y + 10 = 0 using the quadratic formula:

y=(7)±(7)24(1)(10)2(1)y = \frac{-(-7) \pm \sqrt{(-7)^2 - 4(1)(10)}}{2(1)} y=7±49402y = \frac{7 \pm \sqrt{49 - 40}}{2} y=7±92y = \frac{7 \pm \sqrt{9}}{2} y=7±32y = \frac{7 \pm 3}{2}

Thus, y=7+32=5y = \frac{7 + 3}{2} = 5 and y=732=2y = \frac{7 - 3}{2} = 2.

Step 7: Find xx.

Recall that y=log3(x)y = \log_3(x), so:

  • If y=2y = 2, then log3(x)=2\log_3(x) = 2, which means x=32=9x = 3^2 = 9.
  • If y=5y = 5, then log3(x)=5\log_3(x) = 5, which means x=35=243x = 3^5 = 243.

Thus, the solutions are x=9x = 9 and x=243x = 243.

Final Answer:

The solutions are x=9x = 9 and x=243x = 243.


Would you like more details on any of these steps or help with related problems? Here are some related questions for further exploration:

  1. How does the change of base formula work for logarithms?
  2. What is the Rational Root Theorem, and how is it applied to solve cubic equations?
  3. How do you factor cubic polynomials in general?
  4. Can you solve logarithmic equations with different bases?
  5. How can you use logarithms in real-world applications?

Tip: Always check if substituting your solutions back into the original equation satisfies all conditions.

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Math Problem Analysis

Mathematical Concepts

Logarithms
Change of Base Formula
Cubic Equations
Quadratic Formula
Rational Root Theorem

Formulas

log_b(m/n) = log_b(m) - log_b(n)
log_x(3) = 1/log_3(x)
Quadratic Formula: y = (-b ± √(b² - 4ac)) / 2a

Theorems

Change of Base Formula
Rational Root Theorem

Suitable Grade Level

Grades 10-12