Practice Midterm Examination 2 – Math 142B
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Practice Midterm Examination 2 – Math 142B
Question 1. Consider the series
n2 log n . (a) Prove that the series converges. (b) Does the series converge absolutely? |
Solution (a). The series converges since limn→∞ (n + 1)/(n2 log n) = 0, by the alternating series theorem. The series does not converge absolutely.
∞ n + 1
diverges if the following integral
\2 n dx
diverges as n → ∞ . This integral is larger than
\2 n dx = log log x '2(n) = log log n − log log 2
which diverges as n → ∞ .
Question 2. Let f(x) = ex .
(a) Determine the nth Bernstein polynomial fn (x).
(b) Does fn (x) converge uniformly to f(x) for x ≥ 0?
Solution (a).
fn (x) =对(b) ek/n ( )k(n) xk (1 − x)n−k = (1 − x)n 对(n) ( ) (k(n) )k
and by the binomial theorem this is
(1 − x)n (1 + e1/nx/(1 − x))n = (1 − x + xe1/n)n . Solution (b). The convergence is not uniform – see notes in Lecture 3.
Question 3. Determine the Taylor series for f(x) = arctanx about x = 0 together with the radius of convergence of the Taylor series. Does the series converge at the endpoints of the interval of convergence?
Solution. Using Taylor’s Theorem, and recalling f\ (x) = 1/(1+x2 ), we have
f(x) =
for −1 < x < 1, and the radius of convergence is r = 1. The series converges at x = 1 and at x = −1. We will go over the details in class.
2022-08-18