Precalculus : Pre-Calculus

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #2 : Exponential Equations And Inequalities

Solving an exponential equation.

Solve

\(\displaystyle 10^{3x}=100\)

Possible Answers:

\(\displaystyle x=\frac{2}{3}\)

\(\displaystyle x=10\)

\(\displaystyle x=\frac{3}{2}\)

\(\displaystyle x=2\)

\(\displaystyle x=3\)

Correct answer:

\(\displaystyle x=\frac{2}{3}\)

Explanation:

Use \(\displaystyle log\) (which is just \(\displaystyle log_{10}\), by convention) to solve.

\(\displaystyle log(10^{3x})=log(100)\)

\(\displaystyle log(10^{3x})=log(10^2)\)

\(\displaystyle 3x=2\)

\(\displaystyle x=\frac{2}{3}\).

Example Question #1 : Exponential Equations And Inequalities

Solve the equation for \(\displaystyle x\) using the rules of logarithms.

\(\displaystyle \ln(17x)+\ln\left(\frac{x}{3}\right)+\log_3(x^2)=4\)

Possible Answers:

\(\displaystyle x\approx 75.15\)

\(\displaystyle x=3\)

\(\displaystyle x\approx 1.04\)

\(\displaystyle x=9\)

\(\displaystyle x\approx1.86\)

Correct answer:

\(\displaystyle x\approx 1.04\)

Explanation:

Expanding the logarithms into sums of logarithms will cancel out the first two x terms, resulting in the equation:

 \(\displaystyle \ln(17)+ln(3)+2\log_3(x)=4\)

Combining the first and second terms, then subtracting the new term over will allow you to isolate the variable term.

Divide both sides of the equation by 2, then exponentiate with 3.

Evaluating this term numerically will give the correct answer.

\(\displaystyle 2\log_3(x)=4-\ln(51)\implies \log_3(x)=2-\frac{\ln(51)}{2}\\ \\ \implies x=3^{2-\ln(51)/2}\approx1.04\)

Example Question #2 : Exponential Equations And Inequalities

Solve the following equation:

\(\displaystyle 10^x=175\)

Possible Answers:

\(\displaystyle x\approx0.224\)

\(\displaystyle x\approx4.48\)

\(\displaystyle x\approx22.4\)

\(\displaystyle x=2.24\)

\(\displaystyle x\approx2.24\)

Correct answer:

\(\displaystyle x\approx2.24\)

Explanation:

To solve this equation, recall the following property:

\(\displaystyle log_by=x\)     Can be rewritten as \(\displaystyle b^x=y\)

\(\displaystyle 10^x=175\)

\(\displaystyle log_{10}175=x\)

Evaluate with your calculator to get

\(\displaystyle x\approx2.24\)

Example Question #3 : Use Logarithms To Solve Exponential Equations And Inequalities

Solve 

\(\displaystyle \frac{e^{x^2-2}}{e^{3x}}=e^{2x-6}\).

Possible Answers:

\(\displaystyle x=1$ or $4\)

\(\displaystyle x=4\)

\(\displaystyle x=1\)

\(\displaystyle x=0$ or 1\)

\(\displaystyle x=7$ or $4\)

Correct answer:

\(\displaystyle x=1$ or $4\)

Explanation:

After using the division rule to simplify the left hand side you can take the natural log of both sides.

If you then combine like terms you get a quadratic equation which factors to,

 \(\displaystyle (x-1)(x-4)=0\).

Setting each binomial equal to zero and solving for \(\displaystyle x\) we get the solution to be \(\displaystyle x=1$ or $4\).

Example Question #1 : Use Logarithms To Solve Exponential Equations And Inequalities

Solve for x: \(\displaystyle 3^x=243\)

Possible Answers:

\(\displaystyle x=3\)

\(\displaystyle x=1\)

\(\displaystyle x=5\)

\(\displaystyle x=-1\)

\(\displaystyle x=4\)

Correct answer:

\(\displaystyle x=4\)

Explanation:

\(\displaystyle 3^x=3^4\rightarrow x=4\)

Example Question #4 : Use Logarithms To Solve Exponential Equations And Inequalities

Solve for x in the following equation: \(\displaystyle 3^{5x-3}=9^{2x}\)

Possible Answers:

\(\displaystyle x=0\)

\(\displaystyle x=1\)

\(\displaystyle x=-1\)

\(\displaystyle x=5\)

\(\displaystyle x=3\)

Correct answer:

\(\displaystyle x=3\)

Explanation:

\(\displaystyle 3^{5x-3}=3^{(2)(2x)}\rightarrow 5x-3=4x\rightarrow x=3\)

Example Question #4 : Use Logarithms To Solve Exponential Equations And Inequalities

Solve for x using the rules of logarithms: \(\displaystyle \log_{3}2x-\log_{3}(x-3)=1\)

Possible Answers:

\(\displaystyle x=3\)

\(\displaystyle x=1\)

\(\displaystyle x=4\)

\(\displaystyle x=2\)

\(\displaystyle x=9\)

Correct answer:

\(\displaystyle x=9\)

Explanation:

\(\displaystyle log_3 \frac{2x}{x-3}=1\rightarrow 3^{log_3 \frac{2x}{x-3}}=3^1\rightarrow \frac{2x}{x-3}=3\rightarrow x=9\)

Example Question #5 : Use Logarithms To Solve Exponential Equations And Inequalities

Solve for x: \(\displaystyle \log_{4}(3x-2)=2\)

Possible Answers:

\(\displaystyle x=4\)

\(\displaystyle x=3\)

\(\displaystyle x=6\)

\(\displaystyle x=2\)

\(\displaystyle x=5\)

Correct answer:

\(\displaystyle x=6\)

Explanation:

\(\displaystyle log_4 (3x-2)=2\rightarrow 3x-2=4^2\rightarrow 3x=18\rightarrow x=6\)

Example Question #7 : Use Logarithms To Solve Exponential Equations And Inequalities

Simplify the log expression: \(\displaystyle logx*log2\)

Possible Answers:

\(\displaystyle 2logx\)

Cannot be simplified any further

\(\displaystyle log(x)^2\)

\(\displaystyle log2x\)

\(\displaystyle 2log2x\)

Correct answer:

Cannot be simplified any further

Explanation:

The logarithmic expression is as simplified as can be. 

 

Example Question #1231 : Pre Calculus

Which of the following could NOT be a possible number of solutions of a system of quadratic and linear equations? 

Possible Answers:

0

1

How many times the graphs intersect 

3

Correct answer:

3

Explanation:

Recall that the solution of a system of equations is given by the intersection points of the graphs. Thus, this question is really asking how many times a parabola and a line can intersect. Visualize a parabola and a line. For this purpose, let's say that the parabola is facing up. If a line is drawn horizontally under the vertex, then it would not intersect the parabola at all, so the system would have no solutions. If the line is tangent to, or just skims the edge, of the parabola then it would only intersect once and the system would have one solution. If the line goes straight through the parabola, then it would intersect twice. There is no other option for the orientation of the line and parabola. Thus, there cannot be three solutions. 

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