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Solve each logarithmic equation. Be sure to reject any value of \(x\) that is not in the domain of the original logarithmic expressions. Give the exact answer. Then, where necessary, use a calculator to obtain a decimal approximation, correct to two decimal places, for the solution. $$\log _{2}(x+25)=4$$

Short Answer

Expert verified
The equation has no solution since the calculated \(x\) value does not belong to the domain of the original logarithmic expressions.

Step by step solution

01

Write in exponential form

Transform this logarithmic equation into its exponential form. The basis of logarithm becomes the base of the exponent, 4 becomes the exponent and \(x+25\) is the result. The equation is now: \(2^4 = x+25\).
02

Calculate the exponential form

Calculate the expression on the left side of the equation. Hence, the equation becomes: \(16 = x+25\).
03

Isolate \(x\)

To isolate \(x\), subtract 25 from both sides of the equation. Resulting in: \(16 - 25 = x\).
04

Calculate the value of \(x\)

Solve for \(x\). Therefore, \(x = -9\).
05

Check the domain of the function

The domain of a logarithm function must be greater than zero, \(x>0\). However, our calculated \(x\) value is negative. So \(x=-9\) is not valid and the original equation has no solution.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Exponential Form
Understanding how to transform logarithmic equations into their exponential form is a significant step in mastering logarithms. It is essential to recognize that logarithms are essentially the inverse processes of exponentiation. When an equation is presented in a logarithmic form such as \(\log_b(x) = y\), it indicates that the base \(b\) raised to the power \(y\) equals \(x\), written in exponential form as \(b^y = x\). This transformation is crucial as it can often simplify the process of solving logarithmic equations.

For example, the equation \(\log _{2}(x+25)=4\) initially poses the question, 'To what power must 2 be raised to get \(x+25\)?' By rewriting it in exponential form \(2^4 = x+25\), it becomes much more manageable. We can then compute the value of \(2^4\), which is 16, providing us with a simple linear equation to solve. It's critical to keep the relationship between the base, exponent, and result straight to avoid confusion when moving between the logarithmic and exponential forms.
Domain of Logarithmic Functions
The domain of a logarithmic function such as \(\log_b(x)\), where \(b\) is the base, consists of all positive real numbers. Mathematically, this is expressed as \(x > 0\). The reason only positive numbers are included in the domain is because you cannot take the logarithm of a negative number or zero in the set of real numbers, as such a logarithm does not exist.

When solving logarithmic equations, it’s vital to consider the domain, as omitting this step might lead to misleading or incorrect solutions. Take our example, \(\log _{2}(x+25)=4\); even after solving for \(x\) and finding a mathematical answer, you must check if that solution is within the function's domain. In this case, solving yields \(x = -9\), which is outside the domain of the logarithmic function (since \(x+25\) must be greater than 0). Therefore, despite having found a numerical answer, it cannot be accepted as a valid solution since it doesn’t satisfy the domain conditions.
Solving Logarithms
When it comes to solving logarithmic equations, the goal is to isolate the logarithmic part of the equation and then convert the equation into exponential form to find the value of its argument. Here are the essential steps typically followed when solving logarithmic equations:

  • Isolate the logarithm, if necessary, by using algebraic operations.
  • Transform the isolated logarithm into an exponential form, utilizing the definition that \(\log_b(x) = y\) implies \(b^y = x\).
  • Solve the resulting equation for the variable of interest.
  • Check that the solution is within the domain of the original logarithmic function – this often-forgotten step is vital to validating the solution.

For instance, with the equation \(\log _{2}(x+25)=4\), after converting to exponential form, you solve for \(x\) straightaway, obtaining \(x = -9\). However, since this solution falls outside the permissible domain \(x > 0\), it’s not a viable answer for a logarithmic equation. Always remember, solutions only make sense if they comply with the constraints set by the logarithmic function's domain.

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Most popular questions from this chapter

Will help you prepare for the material covered in the next section. Solve: \(x(x-7)=3\)

The formula $$t=\frac{1}{c}[\ln A-\ln (A-N)]$$ describes the time, \(t,\) in weeks, that it takes to achieve mastery of a portion of a task, where \(A\) is the maximum learning possible, \(N\) is the portion of the learning that is to be achieved, and \(c\) is a constant used to measure an individual's learning style. a. Express the formula so that the expression in brackets is written as a single logarithm. b. The formula is also used to determine how long it will take chimpanzees and apes to master a task. For example, a typical chimpanzee learning sign language can master a maximum of 65 signs. Use the form of the formula from part (a) to answer this question: How many weeks will it take a chimpanzee to master 30 signs if \(c\) for that chimp is \(0.03 ?\)

Determine whether each equation is true or false. Where possible, show work to support your conclusion. If the statement is false, make the necessary change(s) to produce a true statement. $$\log _{6}\left(\frac{x-1}{x^{2}+4}\right)=\log _{6}(x-1)-\log _{6}\left(x^{2}+4\right)$$

Use your graphing utility to graph each side of the equation in the same viewing rectangle. Then use the \(x\) -coordinate of the intersection point to find the equation's solution set. Verify this value by direct substitution into the equation. $$\log (x+3)+\log x=1$$

Find \(\ln 2\) using a calculator. Then calculate each of the following: \(1-\frac{1}{2} ; \quad 1-\frac{1}{2}+\frac{1}{3} ; \quad 1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}\) \(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5} ; \ldots .\) Describe what you observe.

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