Lesson 3.4, page 410 Exponential & Logarithmic Equations

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Lesson 3.4, page 410 Exponential & Logarithmic Equations

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Lesson 3.4, page 410 Exponential & Logarithmic Equations

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Lesson 3.4, page 410Exponential & Logarithmic Equations

Objective: To solve exponential and logarithmic equations.

- This skill can be used to solve many types of real-world problems, including those involving animal population.

- If bM = bN , then M = N.
STEPS

- Re-write the equation in for form bM = bN.
- Set M = N.
- Solve for the variable.

- Check Point 1: Solve
- a) 5 3x – 6 = 125b) 8 x + 2 = 4 x - 3

- How do we solve an exponential equation in which we can’t re-write with the same base?
- Answer: take the natural or common logarithm of each side

- Isolate the exponential expression.
- Take the natural logarithm on both sides.
- Simplify using one of the following properties:ln b x = x ln b or ln e x = x.
- Solve for the variable.

- Check Point 2: Solve 5x = 134.

- Check Point 3: Solve 7e 2x – 5 = 58.

- Exponential equation – form b cx = a; (an equation with a variable in the exponent)
Example: 73x = 20

Property of Logarithmic Equality

For any M > 0, N > 0, a > 0, and a 1,

loga M = loga N

M = N

- In other words, take the log of both sides and solve.

Solve: 2x = 50

This is an exact answer. We cannot simplify further, but we can approximate using a calculator.

x 5.6439

We can check by finding 25.6439 50.

- Find each value below and compare.

- Check Point 4: Solve 32x - 1 = 7x + 1.

- Check Point 5: Solve e2x – 8ex + 7 = 0.

- A) 3x = 4 B) 62x = 21

C) 3x+4 = 101 D) e0.25w = 12

- Equations containing variables in logarithmic expressions, such as
log2 x = 16 and log x + log (x + 4) = 1,

are called logarithmic equations.

- Obtain a single logarithmic expression on one side.
- Write an equivalent exponential equation.
- Solve and check.

- Check Point 6
a) log2 (x – 4) = 3 b) 4 ln(3x) = 8

- Check Point 7
Solve: log x + log (x – 3) = 1.

Solve: log4x = 3

Solve:

Check: For x = 3:

For x = 3:

Negative numbers do not have real-number logarithms. The solution is 3.

- Get a single log on each side.
- Use the property:
If log b M = log b N, then M = N.

- Solve for the variable.

- Check Point 8
Solve ln (x - 3) = ln (7x – 23) – ln (x + 1).

- Solve:

- Check Point 9: Use the formula in example 9 to answer the question: “What blood alcohol concentration corresponds to a 7% risk of a car accident? (In many states, drivers under the age of 21 can lose their licenses for dring at this level.)

- Check Point 10: How long, to the nearest tenth of a year, will it take $1000 to grow to $3600 at 8% annual interest compounded quarterly?

- Check Point 11: Use the function in Example 11 to find in which year there will be 210 million Internet users in the United States.

a) 75x = 3000b) 52x = 16

c) log(7 – 2x) = -1

d) log 6 – log 3x = -2