The geometric sequence aₙ = 20·0.9ⁿ⁻¹ decreases. Find the first index n for which aₙ < 0.1.
Sequences & series worksheet with answers
1.Easy
a₁ = 20r = 0.9ε = 0.1
n =
2.Easy
The arithmetic sequence −8, −11, −14, −17, … ends with the term −86. How many terms n does it have?
a₁ = −8d = −3aₙ = −86
n =
3.Medium
Find the radius of convergence R of the power series Σk=1∞ 9ᵏx²ᵏ/k.
Σ 9ᵏx²ᵏ/kx₀ = 0
R =
4.Medium
The power series Σk=1∞ (2k)!·(x − 2)ᵏ/(k!)² converges on an interval around x₀ = 2. Find the left end x of that interval.
Σ (2k)!·(x − 2)ᵏ/(k!)²x₀ = 2
x =
5.Hard
Recognise the series and find its sum: S = Σk=0∞ 2ᵏ/k!.
ak = 2ᵏ/k!
S =
6.Hard
f(x) = sin x is approximated at x = 0.5 by its Maclaurin polynomial of degree n. With the Lagrange bound M·|x|ⁿ⁺¹/(n + 1)! and M = 1, find the smallest degree n that guarantees an error below 0.0005.
f(x) = sin xx = 0.5M = 1ε = 0.0005
n =
Answer key
- 1. n = 52
- 2. n = 27
- 3. R = 0.3333
- 4. x = 1.75
- 5. S = 7.389
- 6. n = 4
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