Question

Which of the following series are convergent?

I and II
II and III
All of them
None of them

Answer 1

All of them

Explanation:

According to the ratio test, for a series ∑aₙ:

If lim(n→∞) |aₙ₊₁ / aₙ| < 1, then ∑aₙ converges.

If lim(n→∞) |aₙ₊₁ / aₙ| > 1, then ∑aₙ diverges.

(I) aₙ = 10 / n!

lim(n→∞) |(10 / (n+1)!) / (10 / n!)|

lim(n→∞) |(10 / (n+1)!) × (n! / 10)|

lim(n→∞) |n! / (n+1)!|

lim(n→∞) |1 / (n+1)|

0 < 1

This series converges.

(II) aₙ = n / 2ⁿ

lim(n→∞) |((n+1) / 2ⁿ⁺¹) / (n / 2ⁿ)|

lim(n→∞) |((n+1) / 2ⁿ⁺¹) × (2ⁿ / n)|

lim(n→∞) |(n+1) / (2n)|

1/2 < 1

This series converges.

(III) aₙ = 1 / (2n)!

lim(n→∞) |(1 / (2(n+1))!) / (1 / (2n)!)|

lim(n→∞) |(1 / (2n+2)!) × (2n)! / 1|

lim(n→∞) |(2n)! / (2n+2)!|

lim(n→∞) |1 / ((2n+2)(2n+1))|

0 < 1

This series converges.