Week 7 Practice Assignment

Week 7 Practice Assignment


Mathematics I


Question 1: Function Behavior from Graph

Statement: From the graph, clearly the values of the function tend to 0 as $ x $ tends to $ \infty $. The graph is smooth (no sharp edges or sharp turns) and so the graph has a unique tangent at all points (at $ x = 1 $ and $ x = -1 $ too). The function values are decreasing (slope of the curve is negative) in $[-0.5, 0]$ (Option 7).

Solution: Based on the graph description:

  • Option 2: Values tend to 0 as $ x \to \infty $.
  • Option 5: Graph is smooth and has a unique tangent at all points.
  • Option 7: Function is decreasing in $[-0.5, 0]$ (slope is negative).

Question 2: Convergence of Sequences

Statement: Statement 1: If ${a_n}$ converges to $a$ and ${b_n}$ converges to $b$, then ${a_n b_n}$ converges to $ab$. Statement 2: If $a_n = (-1)^n$, $b_n = (-2)^n$, then ${a_n}$ and ${b_n}$ do not converge. But ${a_n b_n^3} = {(-1)^n (-2)^{3n}} = {8^n}$, which diverges. (Note: The original statement appears to have a typo or error in interpretation.) Statement 3: $\lim a_n = \lim C = C$ where $C$ is a constant.

Solution:

  • Statement 1: True.
  • Statement 2: False as written, but the intended logic is unclear; ${a_n b_n^3}$ actually diverges, not converges to 2.
  • Statement 3: True.

Final Answer (as per PDF): All 3 statements are true (but this may be a misprint or error in the PDF, as Statement 2 is not true as written).


Question 3: Subsequences and Limits

Statement: Note that $\left{\frac{2n+1}{(2n+1)!}\right}$ and $\left{\left(\frac{3n}{2n+1}\right)\right}$ are subsequences of ${c_n}$ and hence have the same limit as ${a_n}$.

Solution:

$$ \lim_{n \to \infty} \frac{2n+1}{(2n+1)!} = 0 $$$$ \lim_{n \to \infty} \frac{3n}{2n+1} = \frac{3}{2} $$

But the PDF suggests: $\lim c_n = 1$ (this may refer to a different sequence or context; the answer is unclear from the given text).


Question 4: Subsequence Identification

Statement: $a_n = \frac{1}{n}$ is a subsequence of ${ \ldots }$. Only one sequence is a subsequence of ${ \ldots }$. $c_n = 0$ cannot be a subsequence of ${ \ldots }$.

Solution: The question is unclear, but the main point is that only one of the given sequences can be a subsequence of another given sequence.


Question 5: Limit of Sequence

Statement: $a_n = \frac{n}{n} = 1$ converges to 1. $b_n = n^2 + 5n - 1$

$$ \lim_{n \to \infty} b_n = \infty $$

But the PDF states:

$$ \lim_{n \to \infty} (n^2 + 5n - 1) = \lim_{n \to \infty} (n^2 + 5n^2 - 1) \quad (\text{incorrect simplification}) $$

and then gives a wrong calculation:

$$ = (1^3 + 5(1) - 1) = 5 $$

This is incorrect. Correct answer: $\lim_{n \to \infty} (n^2 + 5n - 1) = \infty$


Question 6: Limits of Sequences

Statement: $a_n = \frac{2n+1}{3n+2}$

$$ \lim a_n = \lim_{n \to \infty} \frac{2n+1}{3n+2} = \frac{2}{3} $$

$b_n = 27a_n + 9a_n + \frac{2n^2}{n^3+1} \quad (\text{unclear expression})$ PDF states:

$$ \lim b_n = 27(\frac{2}{3}) + 9(\frac{2}{3}) + 0 = 18 + 6 + 0 = 24 $$

But the calculation is inconsistent with the expression. Likely intended: If $b_n = 27a_n + 9a_n + \frac{2n^2}{n^3+1}$, then

$$ \lim b_n = 27 \times \frac{2}{3} + 9 \times \frac{2}{3} + 0 = 18 + 6 + 0 = 24 $$

But the PDF says the answer is 18, which does not match.


Question 7: Unique Tangent Condition

Statement: If there is no sharp turn at $a$, then the graph has a unique tangent at $a$. Curves 1, 3, and 4 have sharp turns at 0 (Curve 4 has a jump at 0). Hence these three curves do not have a unique tangent at 0.

Solution: Only curves without sharp turns or jumps have a unique tangent at 0.


Question 8: Cost Calculation

Statement: $P_1(2.6) = (100 \times 2) + 200 = 400$ $P_2(2.6) = (100 \times 3) + 200 = 500$ $P_3(2.6) = (100 \times 2.6) + 200 = 460$ The lowest cost is Rs 400 by availing scheme A.

Solution: Lowest cost is Rs 400 (Scheme A).


Question 9: Cost Schemes and Tangents

Statement: Scheme A, B, C costs are shown. Clearly, tangents of $p_1(t)$ and $p_2(t)$ do not exist at $t = 1, 2, 3, \ldots$ (jumps). Tangent exists for all values of $t$ for $p_3(t)$.

Solution: Options 1, 2, 4 are right (tangents do not exist at jumps, exist for all $t$ for $p_3(t)$).


Question 10: Piecewise Function

Statement: Figure 2: $p_3(0) = 200$, $p_3(0.5) = 300$, etc. (No question is explicitly asked here.)

Solution: No explicit question or solution is provided for this.


Summary Table

QuestionStatement/ContextSolution/Answer
1Graph behaviorOptions 2,5,7
2Sequence convergenceAll statements true (likely error in Statement 2)
3Subsequence limitUnclear, likely 1
4SubsequenceOnly one is a subsequence
5Sequence limit$a_n \to 1$, $b_n \to \infty$ (PDF has error)
6Sequence limits$a_n \to \frac{2}{3}$, $b_n \to 24$ (PDF: 18, error)
7Unique tangentOnly smooth curves have unique tangent
8Cost calculationLowest cost: Rs 400 (Scheme A)
9Cost schemesOptions 1,2,4 right
10Piecewise functionNo explicit question

Note: Some questions and solutions are ambiguous or contain errors as per the PDF text. The above is a faithful extraction based on the provided content1. There are no further explicit questions or solutions in the document beyond what is listed.


  1. Week-7-practice-assignment-solution.pdf ↩︎