Calculus 2 : Lagrange Error

Study concepts, example questions & explanations for Calculus 2

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Example Questions

Example Question #61 : Taylor Series

Let \displaystyle P_{5}(x) be the fifth-degree Taylor polynomial approximation for \displaystyle f(x) = sin(x), centered at \displaystyle x = 0.

What is the Lagrange error of the polynomial approximation to \displaystyle sin(1)?

Possible Answers:

\displaystyle \frac{1}{7!}

\displaystyle 1

\displaystyle \frac{1}{6!}

\displaystyle 0

\displaystyle \frac{1}{5!}

Correct answer:

\displaystyle \frac{1}{7!}

Explanation:

The fifth degree Taylor polynomial approximating \displaystyle sin(x) centered at \displaystyle x=0 is: 

\displaystyle sin(x) \approx x - \frac{x^3}{3!} + \frac{x^5}{5!}

The Lagrange error is the absolute value of the next term in the sequence, which is equal to \displaystyle \left |\frac{x^7}{7!} \right |.

We need only evaluate this at \displaystyle x=1 and thus we obtain \displaystyle \frac{1^7}{7!} = \frac{1}{7!}

Example Question #1 : Alternating Series With Error Bound

Which of the following series does not converge?

Possible Answers:

\displaystyle \sum_{i=0}^{\infty} \frac{n - 1}{ n^3 + 1}

\displaystyle \sum_{i=0}^{\infty} \frac{(-1)^n}{n}

\displaystyle \sum_{i=0}^{\infty} \frac{n!}{n^2 cos(n)}

\displaystyle \sum_{i=0}^{\infty} \frac{n^2 ln (n)}{n!}

\displaystyle \sum_{i=0}^{\infty} 0.9999999999999^n

Correct answer:

\displaystyle \sum_{i=0}^{\infty} \frac{n!}{n^2 cos(n)}

Explanation:

We can show that the series \displaystyle \sum_{i=0}^{\infty} \frac{n!}{n^2 cos(n)}  diverges using the ratio test.

 

\displaystyle L = \lim_{n ->\infty } \frac{a_{n+1}}{a_{n}} = \lim_{n ->\infty } \left[\left(\frac{(n+1)!}{(n+1)^2 cos (n+1)} \div \frac{n!}{n^2 cos (n) }\right)\right]

 

\displaystyle = \lim_{n ->\infty} \frac{n^2(n+1)cos(n)}{(n+1)^2cos(n+1)} = \lim_{n ->\infty} \frac{n^2cos(n)}{(n+1)cos(n+1)}

 

\displaystyle n^2 will dominate over \displaystyle (n+1) since it's a higher order term. Clearly, L will not be less than, which is necessary for absolute convergence. 

Alternatively, it's clear that \displaystyle n! is much greater than \displaystyle n^2, and thus having \displaystyle n! in the numerator will make the series diverge by the \displaystyle n^{th} limit test (since the terms clearly don't converge to zero).

The other series will converge by alternating series test, ratio test, geometric series, and comparison tests.

 

 

 

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