Proving Limit of x^4 as x→p is p^4

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In summary, if you know delta is smaller than |p|, you can bound the (x+p)(x^2+p^2) term using an upper bound of 15p^3.
  • #1
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how would i prove that lim of x^4 as x->p is p^4? x^4-p^4 = (x-p)(x+p)(x^2+p^2). I'm having trouble controlling the (x+p)(x^2+p^2) term without having to resort to proving for p > 0 and p < 0 seperately.
 
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  • #2
A standard idea is to place an upper bound on your delta to restrict x, then use this to bound the (x+p)(x^2+p^2) part. If you knew delta<=|p|, can you find an upper bound for |(x+p)(x^2+p^2)|?

This is assuming p is not 0. You could modify the delta above to something that would work in this case, or just deal with it seperately.
 
  • #3
I get |x+p| < |3p| and |x^2+p^2| < 5p^2 if delta < |p|. So then 15p^3 is an upper bound. Is this correct? And as far as dealing with p = 0, can I just add one to the denominator? How did you come up with using |p| for delta?
 
  • #4
That looks good for an upper bound. If p=0, you can't take |p| as an upper bound for delta, you'd want something like |p|+1, but this makes everything else slightly uglier (but perfectly doable). Taking |p| as an upper bound for delta was pretty arbitrary, it just made sure x was positive (or negative if p was negative) and this looked a little cleaner. You could just as well take 1 as an upper bound for delta here.

In general you want to make sure your delta is small enough that the piece you're trying to bound is actually bounded on |x-p|<delta, usually you're trying to avoid zeros in the denominator or where the function otherwise "blows up"
 
  • #5
Thanks a lot for the help. I guess I had it right before, but it was really a mess without using |p| as the delta upper. as you said, this makes the upper bound much cleaner. For p=0, i just used delta = (epsilon)^(1/4).
 

1. What does it mean to prove the limit of x^4 as x approaches p is p^4?

When we say that the limit of x^4 as x approaches p is p^4, we mean that as x gets closer and closer to the value of p, the value of the expression x^4 also gets closer and closer to the value of p^4. In other words, the limit is the value that the function approaches as x gets infinitely close to p.

2. How do you prove the limit of x^4 as x approaches p is p^4?

To prove this limit, we need to show that as x gets arbitrarily close to p, the difference between the value of x^4 and p^4 approaches 0. This can be done by using the formal definition of a limit, which involves using epsilon-delta notation and showing that for any positive value of epsilon, we can find a corresponding value of delta that satisfies the definition.

3. What is the significance of proving the limit of x^4 as x approaches p is p^4?

Proving this limit is important because it helps us understand the behavior of the function x^4 as x gets closer to p. It also allows us to make accurate predictions and calculations involving this function, which can have real-world applications in fields such as physics, engineering, and economics.

4. What are some common techniques used to prove limits?

Some common techniques used to prove limits include algebraic manipulation, substitution, and the use of limit laws such as the sum, difference, product, and quotient rules. In more advanced cases, we may also use L'Hôpital's rule, squeeze theorem, or Taylor series to prove limits.

5. Can the limit of x^4 as x approaches p be proven using a graph?

No, a graph alone is not enough to prove the limit of x^4 as x approaches p. While a graph can help us visualize the behavior of the function, it cannot provide a formal proof. To prove a limit, we need to use mathematical techniques and definitions, as well as logical reasoning.

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