Spivak's Calculus - Problem 1.4(xii) [exponential inequality]

In summary, the task is to find all solutions for the inequality x+3^x<4 and the general problem x+a^x<b. It is not possible to isolate x algebraically in the general case, but it can be solved by guessing the intersection point of the graphs of x+3^x and b.
  • #1
middleCmusic
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Homework Statement



The task is to find all solutions of the following inequality:

[itex] x+3^x <4 [/itex]

But I was trying to find a solution for this problem in general:

[itex] x+a^x < b [/itex]

Homework Equations



n/a

The Attempt at a Solution



[itex] a^x < b-x [/itex]

[itex] \text{log}_a(a^x) < \text{log}_a (b-x) [/itex]

[itex] x < \text{log}_a(b-x) [/itex]

I can't see how to isolate [itex]x[/itex]...

[Context: I'm going through Spivak for self-study to patch up holes in my understanding (and for fun). I'm a 3rd-year undergrad so this should be easy for me, but I can't figure this one out. :confused: ]
 
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  • #2
You don't want to go to the general case and try to isolate x. You can't do it. You do want to realize that x+3^x is an increasing function. Where does it equal 4? Solve that by guessing.
 
  • #3
Dick said:
You don't want to go to the general case and try to isolate x. You can't do it. You do want to realize that x+3^x is an increasing function. Where does it equal 4? Solve that by guessing.

Yes, this is the obvious route here.

I believe he was asking about the problem in general though. As in what if you we're trying to solve ##x + a^x < b##. I looked at it for a moment and realized there is indeed no way to solve it algebraically unless you're able to find where the graphs intersect. This requires there to be some numbers involved sadly otherwise we can't really do anything at all as you have to guess at the intersection point.
 

FAQ: Spivak's Calculus - Problem 1.4(xii) [exponential inequality]

What is Spivak's Calculus - Problem 1.4(xii)?

Spivak's Calculus - Problem 1.4(xii) is a problem from the textbook "Calculus" written by Michael Spivak. It is a question that involves solving an exponential inequality.

What is an exponential inequality?

An exponential inequality is an inequality where one or both sides contain exponential expressions. These types of inequalities involve solving for a variable that is in the exponent.

What is the purpose of Problem 1.4(xii) in Spivak's Calculus?

The purpose of Problem 1.4(xii) is to test the reader's understanding of solving exponential inequalities using basic algebraic manipulations.

How do you solve Problem 1.4(xii) in Spivak's Calculus?

To solve Problem 1.4(xii), you first isolate the exponential term on one side of the inequality. Then, you can take the logarithm of both sides to eliminate the exponent and solve for the variable. Finally, you check the solution by plugging it back into the original inequality.

Why is it important to understand exponential inequalities?

Understanding exponential inequalities is important in many fields of science, including physics, chemistry, and economics. They are used to model growth and decay processes, which are found in various natural and man-made systems. Being able to solve these types of inequalities allows for a better understanding and prediction of these processes.

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