Math Problem Statement
- Show that the sequence {xn} defined by x1 = √ 7 and xn+1 = √ 7 + xn converges to the positive root of the equation x 2 − x − 7 =
Solution
Let's solve the problem by showing that the sequence defined by and converges to the positive root of the equation .
Step 1: Find the roots of the equation
The given equation is:
To find the roots, use the quadratic formula: where , , and . Plugging in these values:
So the roots are:
Since , the positive root is:
Step 2: Show that the sequence converges to
We need to show that .
Assume that the sequence converges to some limit . Then, by the definition of the sequence:
Square both sides:
Rearranging terms gives:
This is exactly the quadratic equation we solved earlier, so must be one of the roots of the equation. Since is positive, and each is defined as a square root (which is positive), the sequence is positive, so the limit must be the positive root:
Step 3: Check if the sequence is increasing and bounded
Let's check whether the sequence is increasing and bounded above by .
Show that the sequence is increasing:
We will show that for all .
Assume . Then: Since is an increasing function (because its derivative ), and , it follows that: So the sequence is increasing.
Show that the sequence is bounded:
Next, we check if the sequence is bounded above by .
If , then:
Thus, the sequence is bounded above by .
Conclusion
Since the sequence is increasing and bounded above, by the Monotone Convergence Theorem, it converges. The limit of the sequence must satisfy the equation , and since the sequence is positive, the limit is the positive root .
Thus, the sequence converges to the positive root of the equation .
Would you like any further details or explanations?
Here are some related questions you might find helpful:
- What is the Monotone Convergence Theorem?
- How do you determine if a sequence is bounded?
- What are the general properties of quadratic equations?
- Can a sequence have more than one limit?
- How does the concept of limits apply in real analysis?
- What are other methods to solve quadratic equations besides the quadratic formula?
- How can you determine the rate of convergence of a sequence?
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Math Problem Analysis
Mathematical Concepts
Sequences
Limits
Quadratic Equations
Formulas
Quadratic formula
Monotone Convergence Theorem
Theorems
Monotone Convergence Theorem
Suitable Grade Level
Advanced High School / Undergraduate
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