Chapter 9: Problem 83
Solve each equation. Give the exact solution and, when appropriate, an approximation to four decimal places. $$ \log (7 y+1)=2 \log (y+3)-\log 2 $$
Short Answer
Expert verified
The exact solutions are \(y = 7\) and \(y = 1\); both satisfy the equation.
Step by step solution
01
Use Properties of Logarithms - Condense the Right Side
Recall the property of logarithms that states \(a \log b = \log b^a\). Apply this to the right side: \(2 \log (y+3) = \log((y+3)^2)\). Now, use the subtraction property of logarithms, \(\log a - \log b = \log\left(\frac{a}{b}\right)\), to rewrite:\(\log((y+3)^2) - \log 2 = \log\left(\frac{(y+3)^2}{2}\right)\). Thus, the equation now is \(\log(7y + 1) = \log\left(\frac{(y+3)^2}{2}\right)\).
02
Eliminate Logarithms - Equate the Arguments
Since the logarithms on both sides are equal, their arguments must also be equal. Hence:\(7y + 1 = \frac{(y+3)^2}{2}\).
03
Clear the Fraction - Multiply Both Sides by 2
To eliminate the fraction, multiply both sides of the equation by 2:\(2(7y + 1) = (y + 3)^2\).Simplifying the left side gives: \(14y + 2 = (y+3)^2\).
04
Expand and Rearrange - Form a Quadratic Equation
Expand \((y+3)^2\): \((y + 3)^2 = y^2 + 6y + 9\).Thus the equation becomes\(14y + 2 = y^2 + 6y + 9\).Reorder to form a quadratic equation:\(y^2 + 6y + 9 - 14y - 2 = 0\).Simplify to:\(y^2 - 8y + 7 = 0\).
05
Solve the Quadratic Equation - Factoring
Factor the quadratic equation:\(y^2 - 8y + 7 = 0\) can be factored as \((y - 7)(y - 1) = 0\).Thus, the solutions are \(y = 7\) and \(y = 1\).
06
Verify the Solutions - Check Within Original Equation
Substitute \(y = 7\) into the original logarithmic equation to check:\(\log(7 \times 7 + 1) = \log(50)\), and check \(2 \log(7 + 3) - \log 2 = \log\left(\frac{100}{2}\right) = \log(50)\).Substitute \(y = 1\):\(\log(7 \times 1 + 1) = \log(8)\), and check \(2 \log(1 + 3) - \log 2 = \log\left(\frac{16}{2}\right) = \log(8)\).Both solutions verify.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Properties of Logarithms
Understanding the properties of logarithms is a crucial step in solving logarithmic equations effectively. In our exercise, these properties helped us simplify and manipulate the equation.One property we used is the power rule, which states that multiplying a logarithm by a number is equivalent to the logarithm of the power of the number. Formally, it's expressed as:
- \(a \log b = \log b^a\)
- \(\log a - \log b = \log\left(\frac{a}{b}\right)\)
Quadratic Equation
After manipulating the logarithmic equation, we ended up with a quadratic equation. A quadratic equation is a second-degree polynomial equation of the form:
- \(ax^2 + bx + c = 0\)
- \(y^2 - 8y + 7 = 0\)
Factoring
Factoring is a method used to solve quadratic equations by expressing them as a product of two binomials. In essence, an equation like \(y^2 - 8y + 7 = 0\) can be rewritten using factoring as:
- \((y - 7)(y - 1) = 0\)
- If \((y - 7) = 0\), then \(y = 7\)
- If \((y - 1) = 0\), then \(y = 1\)
Verifying Solutions
Once we solve an equation, especially a transformed one, verifying the solutions is crucial to ensure they satisfy the original equation.In this exercise, after finding the possible solutions \(y = 7\) and \(y = 1\), we substitute them back into the original logarithmic equation. This confirms their validity:
- For \(y = 7\), substituting gives \(\log(50) = \log(50)\)
- For \(y = 1\), substituting gives \(\log(8) = \log(8)\)