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Use mathematical induction to prove that 2+6+10+...+(4n+2) = 2n(n+1) for all positive integers n.

Mathematics

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Answer

c

(x−1)−12(x−1)2+13(x−1)3(x-1) - \frac{1}{2}(x-1)^2 + \frac{1}{3}(x-1)^3

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Part b· 4 steps

Mathematical Induction Proof

The statement to prove is 2+6+10+⋯+(4n+2)=2n(n+1)2+6+10+\dots+(4n+2) = 2n(n+1) for all positive integers nn.

First, let's analyze the sum. The terms 2,6,10,…2, 6, 10, \dots form an arithmetic progression with first term a1=2a_1=2 and common difference d=4d=4. The kk-th term of this sequence is ak=a1+(k−1)d=2+(k−1)4=4k−2a_k = a_1 + (k-1)d = 2 + (k-1)4 = 4k-2.
The last term given in the sum is (4n+2)(4n+2). To find out which term this is, we set ak=4n+2a_k = 4n+2:
4k−2=4n+24k-2 = 4n+2
4k=4n+44k = 4n+4
k=n+1k = n+1.
This means the sum is actually the sum of the first (n+1)(n+1) terms of the sequence ak=4k−2a_k = 4k-2.
The sum of the first mm terms of an arithmetic progression is Sm=m2(a1+am)S_m = \frac{m}{2}(a_1 + a_m).
Here, m=n+1m=n+1, a1=2a_1=2, and am=an+1=4(n+1)−2=4n+4−2=4n+2a_m = a_{n+1} = 4(n+1)-2 = 4n+4-2 = 4n+2.
So, the sum is Sn+1=n+12(2+(4n+2))=n+12(4n+4)=n+12⋅4(n+1)=2(n+1)2S_{n+1} = \frac{n+1}{2}(2 + (4n+2)) = \frac{n+1}{2}(4n+4) = \frac{n+1}{2} \cdot 4(n+1) = 2(n+1)^2.

The problem asks to prove 2(n+1)2=2n(n+1)2(n+1)^2 = 2n(n+1).
Let's check this equality:
2(n+1)2=2(n2+2n+1)=2n2+4n+22(n+1)^2 = 2(n^2+2n+1) = 2n^2+4n+2.
2n(n+1)=2n2+2n2n(n+1) = 2n^2+2n.
Since 2n2+4n+2≠2n2+2n2n^2+4n+2 \neq 2n^2+2n (they are only equal if 2n+2=02n+2=0, which means n=−1n=-1, not a positive integer), the statement as written is incorrect.

It is highly probable that there is a typo in the question, and the intended statement to prove is 2+6+10+⋯+(4n+2)=2(n+1)22+6+10+\dots+(4n+2) = 2(n+1)^2. I will proceed with proving this corrected statement.

Statement to prove (corrected): P(n):2+6+10+⋯+(4n+2)=2(n+1)2P(n): 2+6+10+\dots+(4n+2) = 2(n+1)^2 for all positive integers nn.

  1. Base Case (n=1)

    For n=1n=1, the sum is 2+6=82+6 = 8.
    The RHS is 2(1+1)2=2(2)2=2(4)=82(1+1)^2 = 2(2)^2 = 2(4) = 8.
    Since LHS = RHS, P(1)P(1) is true.

  2. Inductive Hypothesis

    Assume P(k)P(k) is true for some positive integer kk.

    2+6+10+⋯+(4k+2)=2(k+1)22+6+10+\dots+(4k+2) = 2(k+1)^2
  3. Inductive Step

    We need to prove P(k+1)P(k+1) is true, i.e.,

    2+6+10+⋯+(4k+2)+(4(k+1)+2)=2((k+1)+1)22+6+10+\dots+(4k+2) + (4(k+1)+2) = 2((k+1)+1)^2

    The next term in the series is 4(k+1)+2=4k+4+2=4k+64(k+1)+2 = 4k+4+2 = 4k+6.
    Consider the LHS of P(k+1)P(k+1):

    [2+6+10+⋯+(4k+2)]+(4k+6)[2+6+10+\dots+(4k+2)] + (4k+6)

    Using the inductive hypothesis, we substitute 2(k+1)22(k+1)^2 for the sum of the first kk terms:

    2(k+1)2+(4k+6)2(k+1)^2 + (4k+6)

    Expand and simplify:

    2(k2+2k+1)+4k+62(k^2+2k+1) + 4k+6 2k2+4k+2+4k+62k^2+4k+2 + 4k+6 2k2+8k+82k^2+8k+8

    Factor out 2:

    2(k2+4k+4)2(k^2+4k+4)

    Recognize the perfect square:

    2(k+2)22(k+2)^2

    Now, consider the RHS of P(k+1)P(k+1):

    2((k+1)+1)2=2(k+2)22((k+1)+1)^2 = 2(k+2)^2

    Since LHS = RHS, P(k+1)P(k+1) is true.

  4. Conclusion

    By the principle of mathematical induction, the statement 2+6+10+⋯+(4n+2)=2(n+1)22+6+10+\dots+(4n+2) = 2(n+1)^2 is true for all positive integers nn.

Part c

(x−1)−12(x−1)2+13(x−1)3(x-1) - \frac{1}{2}(x-1)^2 + \frac{1}{3}(x-1)^3

Taylor Expansion of ln⁡x\ln x about x=1x=1

The Taylor expansion of a function f(x)f(x) about x=ax=a is given by:

f(x)=f(a)+f′(a)1!(x−a)+f′′(a)2!(x−a)2+f′′′(a)3!(x−a)3+…f(x) = f(a) + \frac{f'(a)}{1!}(x-a) + \frac{f''(a)}{2!}(x-a)^2 + \frac{f'''(a)}{3!}(x-a)^3 + \dots

Here, f(x)=ln⁡xf(x) = \ln x and a=1a=1.

  1. Calculate the function and its derivatives.

    f(x)=ln⁡xf(x) = \ln x f′(x)=1xf'(x) = \frac{1}{x} f′′(x)=−1x2f''(x) = -\frac{1}{x^2} f′′′(x)=2x3f'''(x) = \frac{2}{x^3} f(4)(x)=−6x4f^{(4)}(x) = -\frac{6}{x^4}
  2. Evaluate the function and derivatives at x=1x=1.

    f(1)=ln⁡(1)=0f(1) = \ln(1) = 0 f′(1)=11=1f'(1) = \frac{1}{1} = 1 f′′(1)=−112=−1f''(1) = -\frac{1}{1^2} = -1 f′′′(1)=213=2f'''(1) = \frac{2}{1^3} = 2
  3. Substitute these values into the Taylor series formula.

    f(x)=0+11!(x−1)+−12!(x−1)2+23!(x−1)3+…f(x) = 0 + \frac{1}{1!}(x-1) + \frac{-1}{2!}(x-1)^2 + \frac{2}{3!}(x-1)^3 + \dots f(x)=0+1(x−1)−12(x−1)2+26(x−1)3+…f(x) = 0 + 1(x-1) - \frac{1}{2}(x-1)^2 + \frac{2}{6}(x-1)^3 + \dots f(x)=(x−1)−12(x−1)2+13(x−1)3+…f(x) = (x-1) - \frac{1}{2}(x-1)^2 + \frac{1}{3}(x-1)^3 + \dots
  4. Identify the first three non-zero terms.

    The first term f(1)=0f(1)=0 is zero.
    The first non-zero term is (x−1)(x-1).
    The second non-zero term is −12(x−1)2-\frac{1}{2}(x-1)^2.
    The third non-zero term is 13(x−1)3\frac{1}{3}(x-1)^3.

    The first three non-zero terms of the Taylor expansion of ln⁡x\ln x about x=1x=1 are:

    (x−1)−12(x−1)2+13(x−1)3\boxed{(x-1) - \frac{1}{2}(x-1)^2 + \frac{1}{3}(x-1)^3}
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