Math Problem Statement

How many terms of the series \( \sum_{n=1}^{\infty} \frac{(-1)^n}{n^{3/2}} \) are needed to estimate the value to within \( 10^{-6} \)?

Solution

To solve this problem, we need to find the number of terms NN required to estimate the value of the series

n=1(1)nn3/2\sum_{n=1}^{\infty} \frac{(-1)^n}{n^{3/2}}

to within an error of 10610^{-6}.

Since this is an alternating series, we can use the Alternating Series Estimation Theorem. For an alternating series of the form

n=1(1)n1an\sum_{n=1}^{\infty} (-1)^{n-1} a_n

where an>0a_n > 0, the error RNR_N after NN terms is bounded by the next term:

RN=SSNaN+1|R_N| = |S - S_N| \leq a_{N+1}

where SS is the actual sum of the series, and SNS_N is the sum of the first NN terms.

In this case, an=1n3/2a_n = \frac{1}{n^{3/2}}. We want the error to be less than 10610^{-6}, so:

1(N+1)3/2<106\frac{1}{(N+1)^{3/2}} < 10^{-6}

Now, we can solve for NN by rearranging the inequality:

(N+1)3/2>106(N+1)^{3/2} > 10^6

Taking the cube root of both sides:

N+1>(106)2/3N+1 > (10^6)^{2/3}

Calculating (106)2/3(10^6)^{2/3}:

106×23=104=1000010^{6 \times \frac{2}{3}} = 10^4 = 10000

So:

N+1>10000N+1 > 10000

Thus, N>9999N > 9999.

Therefore, we need at least N=9999N = 9999 terms to estimate the value of the series to within 10610^{-6}.

Would you like more details on this solution or have any questions?

Here are five related questions:

  1. How does the Alternating Series Estimation Theorem apply to other series?
  2. Can this estimation method be applied to non-alternating series?
  3. What happens if the convergence rate is slower than 1n3/2\frac{1}{n^{3/2}}?
  4. How would this approach change if we needed an error bound of 10810^{-8} instead?
  5. Could we apply numerical methods to find the sum instead?

Tip: When working with series, check if it’s alternating, as this can provide useful bounds on the error through the Alternating Series Estimation Theorem.

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Math Problem Analysis

Mathematical Concepts

Infinite Series
Alternating Series
Error Bound

Formulas

Alternating Series Error Bound: \( |R_N| = |S - S_N| \leq a_{N+1} \)
Inequality for error tolerance: \( \frac{1}{(N+1)^{3/2}} < 10^{-6} \)

Theorems

Alternating Series Estimation Theorem

Suitable Grade Level

Undergraduate Level