How do you do these three questions?

Step-by-step explanation:

a) Use ratio test.

lim(n→∞)│aₙ₊₁ / aₙ│< 1

lim(n→∞)│[x²ᵏ⁺¹ / (2(k+1))!] / [x²ᵏ / (2k)!]│< 1

lim(n→∞)│[x²ᵏ⁺¹ / (2k+2)!] × [(2k)! / x²ᵏ]│< 1

lim(n→∞)│x (2k)! / (2k+2)!│< 1

lim(n→∞)│x / ((2k+2) (2k+1))│< 1

0 < 1

The interval of convergence is (-∞, ∞).

b) f(x) = ∑₎₍₌₀°° x²ᵏ / (2k)!

f'(x) = ∑₎₍₌₁°° 2k x²ᵏ⁻¹ / (2k)!

f'(x) = ∑₎₍₌₁°° x²ᵏ⁻¹ / (2k−1)!

f'(x) = ∑₎₍₌₀°° x²ᵏ⁺¹ / (2k+1)!

c) f(x) + f'(x) = ∑₎₍₌₀°° x²ᵏ / (2k)! + ∑₎₍₌₀°° x²ᵏ⁺¹ / (2k+1)!

Notice f(x) is the sum of even terms (2k) and f'(x) is the sum of odd terms (2k+1).  Therefore:

f(x) + f'(x) = ∑₎₍₌₀°° xᵏ / k!

f(x) + f'(x) = eˣ

step-by-step explanation:

step-by-step explanation: