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In Exercises 85 - 92, use the One-to-One Property to solve the equation for \( x \). \( \ln(x^2 - x) = \ln 6 \)

Short Answer

Expert verified
The solution to the equation is \( x = 3 \).

Step by step solution

01

Apply the one-to-one property of Logarithms

According to the one-to-one property of logarithms, if \( \ln a = \ln b \), then \( a = b \). So, set the equations inputs equal to each other: \( x^2 - x = 6 \).
02

Simplify the Quadratic Equation

Subtract 6 from both sides of the equations to get it to the standard form of quadratic equation: \( x^2 - x - 6 = 0 \).
03

Factor the Quadratic Equation

The quadratic equation factors to: \( (x-3)(x+2) = 0 \).
04

Solve for x

Setting each factor equal to zero gives the solutions \( x = 3 \) and \( x = -2 \).
05

Check the solutions

We have to ensure that the solutions do not violate the domain of the logarithmic function. The original logarithmic equation \( \ln(x^2 - x) = \ln 6 \) is only valid for \( x^2 - x > 0 \), as the natural logarithm is undefined for zero and negative numbers. Checking for \( x = 3 \), the inequality \( 3^2 - 3 > 0 \) is true. However, for \( x = -2 \), the inequality \(-2^2 - (-2) > 0 \) is false. Therefore, the only valid solution for the given problem is \( x = 3 \).

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Key Concepts

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

One-to-One Property of Logarithms
Understanding the one-to-one property of logarithms is essential when you are tasked with solving logarithmic equations. The one-to-one property simply states that if two logs with the same base are equal, then their inputs must also be equal. This is denoted as if \( \ln(a) = \ln(b) \), then \( a = b \). This property is critical because it allows us to transform a logarithmic equation into a much simpler algebraic equation, making it easier to find the solution for \(x\). In the provided exercise, we used this property to equate \( x^2 - x \) and 6, setting the stage to solve for \(x\).

It is important to realize that this property relies on the assumption that both sides of the equation involve the natural logarithm (ln) of valid inputs—that is, both \(a\) and \(b\) must be positive numbers, as natural logarithms are not defined for zero or negative numbers. In practice, this means that after finding the potential solutions, we still need to check them against the domain of the natural logarithm to ensure they are valid.
Quadratic Equation
A quadratic equation is a second-degree polynomial equation of the form \( ax^2 + bx + c = 0 \), where \(a\), \(b\), and \(c\) are constants and \(a eq 0\). Finding the solutions to a quadratic equation, which are also known as the roots or zeros of the equation, can be approached in several ways, including factoring, using the quadratic formula, or completing the square. The exercise given involves simplifying a logarithmic equation to obtain a quadratic equation, which was then set to the standard form, necessary to begin the solving process.

Solving a quadratic equation reveals the points where the quadratic function crosses the x-axis. These solutions are of immense interest not only in pure algebra but also in various application fields such as physics, engineering, and economics. In our scenario, the equation \( x^2 - x - 6 = 0 \) had to be solved to find the value of \(x\) that satisfies the original logarithmic equation.
Factoring Quadratics
Factoring is a powerful method to solve quadratic equations when they can be broken down into products of binomials. To factor a quadratic, you seek two binomials that when multiplied together produce the original quadratic equation. This usually requires finding two numbers that add up to the coefficient of the middle term, and multiply to the constant term. In the provided example, the quadratic equation \( x^2 - x - 6 = 0 \) is factored into \((x-3)(x+2) = 0\).

The Zero Product Property then comes into play, which asserts that if a product of two factors equals zero, at least one of the factors must be zero. This means we can set each binomial equal to zero and solve for \(x\). By using this technique, we can quickly pinpoint the solutions to the equation. Remember, though, each solution must be checked to ensure it fits within the domain of the original logarithmic equation.
Natural Logarithm Domain
The natural logarithm is a logarithmic function that has a base of \(e\), which is an irrational constant approximately equal to 2.71828. The domain of the natural logarithm function, denoted as \(\ln(x)\), consists of all positive real numbers. This means the input \(x\) must be greater than zero for the function to yield a real number output.

When solving logarithmic equations, it's essential to keep the domain in mind because if a solution does not fall within the domain, it must be discarded as extraneous. In the example problem, the domain of the function dictated that only the positive solution, \(x=3\), was valid since the negative solution, \(x=-2\), would invalidate the original equation by requiring the logarithm of a negative number, which is not defined. Always remember to check your solutions against the domain to ensure they are acceptable for the given logarithmic function.

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Most popular questions from this chapter

If the annual rate of inflation averages \( 4\% \) over the next \( 10 \) years, the approximate costs \( C \) of goods or services during any year in that decade will be modeled by \( C(t) = P(1.04)^t \), where \( t \) is the time in years and \( P \) is the present cost. The price of an oil change for your car is presently \( \$23.95 \). Estimate the price \( 10 \) years from now.

A cup of water at an initial temperature of \( 78^{\circ}C \) is placed in a room at a constant temperature of \( 21^{\circ}C \). The temperature of the water is measured every 5 minutes during a half-hour period.The results are recorded as ordered pairs of the form \( (t \), \( T) \), where \( t \) is the time (in minutes) and \( T \) is the temperature (in degrees Celsius). \( \left(0, 78.0^{\circ}\right) \), \( \left(5 , 66.0^{\circ}\right) \), \( \left(10, 57.5^{\circ}\right) \), \( \left(15 , 51.2^{\circ}\right) \), \( \left(20 , 46.3^{\circ}\right) \), \( \left(25, 42.4^{\circ}\right) \), \( \left(30 , 39.6^{\circ}\right) \) (a) The graph of the model for the data should be asymptotic with the graph of the temperature of the room. Subtract the room temperature from each of the temperatures in the ordered pairs. Use a graphing utility to plot the data points \( \left(t , T\right) \) and \( \left(t, T - 21\right) \). (b) An exponential model for the data \( \left(t, T - 21\right) \) is given by \( T - 21 = 54.4\left(0.964\right)^t \) Solve for \( T \) and graph the model. Compare the result with the plot of the original data. (c) Take the natural logarithms of the revised temperatures. Use a graphing utility to plot the points \( \left(t, In\left(T - 21\right)\right) \) and observe that the points appear to be linear. Use the regression feature of the graphing utility to fit a line to these data. This resulting line has the form \( In\left(T - 21\right) = at + b \). Solve for \( T \), and verify that the result is equivalent to the model in part (b). (d) Fit a rational model to the data. Take the reciprocals of the \( y \)-coordinates of the revised data points to generate the points \( \dfrac{1}{T - 21} = at + b \). Solve for \( T \), and use a graphing utility to graph the rational function and the original data points. (e) Why did taking the logarithms of the temperatures lead to a linear scatter plot? Why did taking the reciprocals of the temperatures lead to a linear scatter plot?

In Exercises 39 - 44, use a graphing utility to construct a table of values for the function. Then sketch the graph of the function. \( f(x) = e^{-x} \)

At \( 8:30 \) A.M., a coroner was called to the home of a person who had died during the night. In order to estimate the time of death, the coroner took the persons temperature twice. At \( 9:00 \) A.M. the temperature was \( 85.7^\circ F \) and at \( 11:00 \) A.M. the temperature was \( 82.8^\circ F \). From these two temperatures,the coroner was able to determine that the time elapsed since death and the body temperature were related by the formula \( t = -10 ln \dfrac{T - 70}{98.6 - 70} where \) t \( is the time in hours elapsed since the person died and \) T \( is the temperature (in degrees Fahrenheit) of the persons body. (This formula is derived from a general cooling principle called Newtons Law of Cooling. It uses the assumptions that the person had a normal body temperature of \) 98.6^\circ F \( at death, and that the room temperature was a constant \) 70^\circ F $. ) Use the formula to estimate the time of death of the person.

In Exercises 81 - 112, solve the logarithmic equation algebraically. Approximate the result to three decimal places. \( \log x = 6 \)

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