What values of x satisfy the inequality x + 3^x < 4?

  • Thread starter zeion
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In summary, the inequality x + 3^x < 4 can be solved by finding values of x less than 1 or greater than 1. Taking the logarithm of the inequality does not provide much insight, and it can also be solved by considering the graphs of y = x and y = 3x, both of which are strictly increasing functions. As x becomes more negative, the function x + 3^x approaches y = x, while as x becomes larger, it approaches y = 3x. Therefore, for x < 1, the inequality holds true, and for x > 1, the inequality does not hold true.
  • #1
zeion
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Homework Statement



For what x does [tex]x + 3^x < 4[/tex]


Homework Equations





The Attempt at a Solution



The thing is = 4 when x is 1. So I want x < 1.
But is there a way to do this algebraically? Like with log or something?
 
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  • #2
Taking a log doesn't really make this problem much more transparent. If this were the Calculus forum, I'd suggest looking at Newton's method for finding a numerical approximation: http://en.wikipedia.org/wiki/Newton's_method
 
  • #3
You can say something about this inequality just by knowing something about the graphs of y = x and y = 3x. Both of these are strictly increasing functions, so their sum is also a strictly increasing function. Let's define f(x) = x + 3x.

Looking at asymptotic behavior, as x gets more and more negative, f(x) approaches the graph of y = x. IOW, for very negative x, f(x) [itex]\approx[/itex] x. As x gets larger and larger, f(x) approaches y = 3x.

You already found out that f(1) = 4, so for any x > 1, then f(x) > 4. Similarly, if x < 1, f(x) < 4.
 

1. What is the purpose of the inequality x + 3^x < 4?

The purpose of this inequality is to determine the values of x that satisfy the given expression. In other words, we are trying to find the values of x that make the inequality true.

2. What is the significance of the exponent in the expression 3^x?

The exponent, x, represents the power to which the base, 3, is raised. In this case, we are interested in finding values of x that make the expression less than 4 when added to x.

3. How can I solve this inequality for x?

To solve this inequality, you can use algebraic techniques such as isolating the variable and factoring. You can also use graphical methods to visually determine the values of x that satisfy the inequality.

4. Are there any restrictions on the values of x in this inequality?

Yes, there are restrictions on the values of x. Since we are dealing with a logarithmic function, x must be a positive real number. Additionally, x cannot be equal to 1 since 3^1 = 3, which would make the expression equal to 4, not less than 4.

5. What are some applications of solving this type of inequality?

Inequalities like this one are commonly used in mathematics and science to model real-life situations such as population growth, compound interest, and radioactive decay. Solving these types of inequalities helps us understand and make predictions about these phenomena.

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