let's finish up Question Five.
Question Five
a) Define the following:
- i. Taylor Series: A Taylor series is a representation of a function as an infinite sum of terms, where each term is calculated from the values of the function's derivatives at a single point. It is given by the formula:
f(x)=∑n=0∞n!f(n)(a)(x−a)n
- ii. Maclaurin Series: A Maclaurin series is a special case of the Taylor series where the expansion point a is 0. It is given by the formula:
f(x)=∑n=0∞n!f(n)(0)xn
b) Find the Maclaurin Series for the function f(x)=ex.
Step 1: Find the derivatives of f(x)=ex and evaluate them at x=0.
f(x)=ex⟹f(0)=e0=1
f′(x)=ex⟹f′(0)=e0=1
f′′(x)=ex⟹f′′(0)=e0=1
In general, for any n≥0:
f(n)(x)=ex⟹f(n)(0)=1
Step 2: Substitute the derivatives into the Maclaurin series formula.
The Maclaurin series is f(x)=∑n=0∞n!f(n)(0)xn.
ex=0!f(0)x0+1!f′(0)x1+2!f′′(0)x2+3!f′′′(0)x3+…
ex=0!1x0+1!1x1+2!1x2+3!1x3+…
ex=1+x+2!x2+3!x3+…
The Maclaurin series for f(x)=ex is n=0∑∞n!xn.
c) Find the Taylor series for the function f(x)=lnx about x=2.
Step 1: Find the derivatives of f(x)=lnx and evaluate them at x=2.
f(x)=lnx⟹f(2)=ln2
f′(x)=x−1⟹f′(2)=2−1=21
f′′(x)=−x−2⟹f′′(2)=−2−2=−41
f′′′(x)=2x−3⟹f′′′(2)=2⋅2−3=82=41
f(4)(x)=−6x−4⟹f(4)(2)=−6⋅2−4=−166=−83
In general, for n≥1:
f(n)(x)=(−1)n−1(n−1)!x−n
f(n)(2)=(−1)n−1(n−1)!2−n
Step 2: Substitute the derivatives into the Taylor series formula about a=2.
The Taylor series is f(x)=∑n=0∞n!f(n)(a)(x−a)n.
f(x)=f(2)+1!f′(2)(x−2)+2!f′′(2)(x−2)2+3!f′′′(2)(x−2)3+…
f(x)=ln2+1!1/2(x−2)+2!−1/4(x−2)2+3!1/4(x−2)3+…
f(x)=ln2+21(x−2)−81(x−2)2+241(x−2)3−…
For n≥1, the general term is n!f(n)(2)(x−2)n=n!(−1)n−1(n−1)!2−n(x−2)n=n⋅2n(−1)n−1(x−2)n.
The Taylor series for f(x)=lnx about x=2 is ln2+n=1∑∞n⋅2n(−1)n−1(x−2)n.
That's all 5 problems for today! Great work. Drop the next question when you're ready.