- Topic
- Taylor polynomials
- Difficulty
- Standard
- Suggested time
- 4 min
- Marks
- 3
Taylor polynomials practice question 9
Find the second-order Taylor polynomial of about .
The centre is , not 0. At which point must each derivative be evaluated, and in powers of what is the polynomial written?
About , . Here .
and . At , and .
A polynomial in x, with exact coefficients. Use ^ for powers and brackets for every input: sin(2x), e^(x^2), log(x+1). Give exact values: 1/6, not 0.167.
Theory UBC Math 100 §9CLP-1 §3.4Key results
Worked solution
Strategy
Find the value and the first two derivatives at the centre π/6, where sine and cosine are exact, and build the polynomial in powers of x − π/6, dividing the second derivative by 2!.
Working
Differentiate as functions first.
Then evaluate at the centre, not at 0.
The square term is f″(π/6)/2! = −1/4. Expanding the brackets gives the same polynomial in another form.
Answer
The answer is T two of x equals one half, plus root 3 over 2 times (x minus pi over 6), minus one quarter of (x minus pi over 6) squared.
Check
At x = 0.5, T₂ and sin 0.5 both come to 0.4794 to four places.
Where this goes wrong
Using the Maclaurin coefficients of sin x, which are 0, 1 and 0, with powers of x − π/6, which gives x − π/6. The coefficients of a Taylor polynomial are the derivatives at its own centre, here π/6.
Leans on
The derivatives of sine and cosine, from Exponential and trigonometric functions
Try a similar question
Find the third-order Taylor polynomial of about .Saved in this browser only.