Then, the value of lim f (n) is equal to : 17100 A 3 + 4/3 * ln(7) 4 - 3/4 * ln(7/3) 4 - 4/3 * ln(7/3) 3 + 3/4 * ln(7) f(n)= n + (16 + 5n - 3n ^ 2)/(4n + 3n ^ 2) + (32 + n - 3n ^ 2)/(8n + 3n ^ 2) + (48 - 3n - 3n ^ 2)/(12n + 3n ^ 2) +***+ 25n-7n^ 2 7n^ 2 .

3 min read
43 views
Published July 11, 2025
Mathematics
Calculus
Limits
Riemann Sums
Definite Integrals

💡 Want to ask your own questions?

Get instant explanations with AI • Free trial

Detailed Explanation

1. Recognising the pattern

Write the function more neatly for a fixed positive integer nn:

f(n)=  n  +k=1n16k+(94k)n3n24kn+3n2.\begin{aligned} f(n)=&\; n \; + \sum_{k=1}^{n}\frac{16k + (9-4k)n - 3n^{2}}{4kn + 3n^{2}}. \end{aligned}

Why does that numerator look so weird? Because the popular trick “add 1 and subtract 1” is hidden inside it!

2. Kill the big–nn explosion

Each term inside the sum is roughly 1-1 (because the 3n2-3n^{2} and 3n23n^{2} dominate). Summing 1-1 over nn terms gives n-n, which cancels the lonely leading +n+n. So everything infinite disappears and you are left with a finite expression:

Tk=16k+(94k)n3n24kn+3n2,Tk+1=16k+9n4kn+3n2.\begin{aligned} T_k &= \frac{16k + (9-4k)n - 3n^{2}}{4kn + 3n^{2}},\\[4pt] T_k+1 &= \frac{16k + 9n}{4kn + 3n^{2}}. \end{aligned}

Therefore

f(n)=k=1n16k+9n4kn+3n2.\boxed{\,f(n)=\sum_{k=1}^{n}\frac{16k + 9n}{4kn + 3n^{2}}\,}.

3. Spotting the Riemann sum

Factor out n2n^{2} from numerator and denominator:

16k+9n4kn+3n2=16kn2+9n4kn+3.\frac{16k + 9n}{4kn + 3n^{2}} = \frac{ \dfrac{16k}{n^{2}} + \dfrac{9}{n} }{\dfrac{4k}{n} + 3}.

Put xk=knx_k = \dfrac{k}{n} (right-end points of nn equal sub-intervals on [0,1][0,1]). Then

16k+9n4kn+3n2=1n16xk+94xk+3.\frac{16k + 9n}{4kn + 3n^{2}} = \frac{1}{n}\,\frac{16x_k + 9}{4x_k + 3}.

Hence

f(n)=k=1n1ng(xk)where g(x)=16x+94x+3. f(n)=\sum_{k=1}^{n}\frac{1}{n}\,g(x_k)\quad\text{where } g(x)=\frac{16x+9}{4x+3}.

This is exactly the right-Riemann sum for

01g(x)  dx.\int_{0}^{1} g(x)\;dx.

4. Compute the integral

Do a quick long division:

16x+94x+3=434x+3.\frac{16x+9}{4x+3}=4-\frac{3}{4x+3}.

So

01(434x+3)dx=4[x]013[14ln(4x+3)]01=434ln(73).\int_{0}^{1}\left(4-\frac{3}{4x+3}\right)dx =4\bigl[x\bigr]_{0}^{1}-3\left[\frac{1}{4}\ln(4x+3)\right]_{0}^{1} =4-\frac{3}{4}\ln\left(\frac{7}{3}\right).

5. Limit value

Therefore

limnf(n)=434ln ⁣(73).\boxed{\displaystyle \lim_{n\to\infty}f(n)=4-\frac{3}{4}\ln\!\left(\frac{7}{3}\right)}.

Among the four options, that matches Option B.

Simple Explanation (ELI5)

What is the question?

You have a long-looking fraction–sum that depends on a number nn. The question asks, “When nn becomes a really, really huge number, what does the whole expression settle down to?”

How to think about it (like a story)

  1. Picture nn as the number of tiny steps along a road from 0 to 1.
  2. Each fraction in the sum is like measuring the height of a curve at one of those steps.
  3. Adding all those little heights and multiplying by the step–width (1/n1/n) is the same trick we use to turn a sum into an area under a curve (this is what grown-ups call a Riemann sum).
  4. When nn is gigantic, the road has millions of tiny steps, so the sum is almost exactly the integral of that curve from 0 to 1.
  5. Once you spot the right curve, doing the integral is easy and gives a nice clean number with a small natural‐log piece.

That final number is the answer!

👆 Found this helpful? Get personalized explanations for YOUR questions!

Step-by-Step Solution

Step–by–step calculation

  1. Rewrite f(n)f(n)

    f(n)=n+k=1n16k+(94k)n3n24kn+3n2.f(n)=n+\sum_{k=1}^{n}\frac{16k+(9-4k)n-3n^2}{4kn+3n^2}.
  2. Separate out the “+1+1

    Tk=16k+(94k)n3n24kn+3n2,Tk+1=16k+9n4kn+3n2.\begin{aligned} T_k &= \frac{16k+(9-4k)n-3n^2}{4kn+3n^2},\\ T_k+1 &= \frac{16k+9n}{4kn+3n^2}. \end{aligned}

    Hence

    f(n)=nn+k=1n16k+9n4kn+3n2=k=1n16k+9n4kn+3n2.f(n)=n-n+\sum_{k=1}^{n}\frac{16k+9n}{4kn+3n^2}=\sum_{k=1}^{n}\frac{16k+9n}{4kn+3n^2}.
  3. Normalise each term
    Divide top and bottom by n2n^{2} and set xk=knx_k=\dfrac{k}{n}:

    f(n)=k=1n1n16xk+94xk+3.f(n)=\sum_{k=1}^{n}\frac{1}{n}\,\frac{16x_k+9}{4x_k+3}.
  4. Recognise the Riemann sum
    As nn\to\infty this becomes the integral

    0116x+94x+3dx.\int_{0}^{1}\frac{16x+9}{4x+3}\,dx.
  5. Integrate
    Long-divide:

    16x+94x+3=434x+3.\frac{16x+9}{4x+3}=4-\frac{3}{4x+3}.

    Then

    01(434x+3)dx=4[x]0134[ln(4x+3)]01=434ln(73).\begin{aligned} \int_{0}^{1}\left(4-\frac{3}{4x+3}\right)dx&=4\left[x\right]_{0}^{1}-\frac{3}{4}\left[\ln(4x+3)\right]_{0}^{1}\\[6pt] &=4-\frac{3}{4}\ln\left(\frac{7}{3}\right). \end{aligned}
  6. Final answer

    limnf(n)=434ln ⁣(73).\boxed{\lim_{n\to\infty}f(n)=4-\frac{3}{4}\ln\!\left(\frac{7}{3}\right)}.

    This is Option (2) (or B) from the list.

Examples

Example 1

Estimating the area under a speed–time graph by adding many thin rectangles (Riemann sum) and then shrinking their width to zero (integral).

Example 2

In probability, converting the sum of binomial probabilities into an integral to approximate a normal distribution (De Moivre–Laplace).

Example 3

Using long division to integrate rational functions in electronics when finding the average power over one period of a signal.

Visual Representation

References

🤔 Have Your Own Question?

Get instant AI explanations in multiple languages with diagrams, examples, and step-by-step solutions!

AI-Powered Explanations
🎯Multiple Languages
📊Interactive Diagrams

No signup required • Try 3 questions free