Question

Please help!

Find the interval of convergence for:

Please note: The 2 in the picture must actually also be an exponent, so n^2^n.

Answer 1

-∞ < x < ∞

Explanation:

∑ xⁿ / n^(2ⁿ)

Apply ratio test:

lim(n→∞) | [xⁿ⁺¹ / (n+1)^(2ⁿ⁺¹)] / [xⁿ / n^(2ⁿ)] |

lim(n→∞) | [xⁿ⁺¹ / (n+1)^(2ⁿ⁺¹)] × [n^(2ⁿ) / xⁿ] |

lim(n→∞) | x n^(2ⁿ) / (n+1)^(2ⁿ⁺¹) |

0

The series converges for all values of x.