Q54
jee_main_2025_02_april_evening
Relations
Let mathrmA = \1, 2, 3, dots, 100\$\mathrm{A} = \{1, 2, 3, \dots, 100\}$ and mathrmR$\mathrm{R}$ be a relation on mathrmA$\mathrm{A}$ such that mathrmR = \(a, b) : a = 2b + 1\$\mathrm{R} = \{(a, b) : a = 2b + 1\}$. Let (a_1, a_2), (a_2, a_3), (a_3, a_4), dots, (a_k, a_k+1)$(a_1, a_2), (a_2, a_3), (a_3, a_4), \dots, (a_k, a_{k+1})$ be a sequence of k$k$ elements of mathrmR$\mathrm{R}$ such that the second entry of an ordered pair is equal to the first entry of the next ordered pair. Then the largest integer k$k$, for which such a sequence exists, is equal to:
Solution
### Related Formula
textChain definition: a_i = 2 a_i+1 + 1 quad textfor i = 1, 2, dots, k$$\text{Chain definition: } a_{i} = 2 a_{i+1} + 1 \quad \text{for } i = 1, 2, \dots, k$$
### Core Logic
To find the longest sequence of connected pairs, we trace the relation backward starting from the smallest elements in A$A$.
### Step 1: Trace the relations backward
To maximize k$k$, we want the chain of elements to go down as low as possible. Let the final element in the chain be a_k+1 in mathrmA$a_{k+1} \in \mathrm{A}$.
Since a_k = 2 a_k+1 + 1$a_k = 2 a_{k+1} + 1$:
- If a_k+1 = 1 implies a_k = 3$a_{k+1} = 1 \implies a_k = 3$
- If a_k+1 = 2 implies a_k = 5$a_{k+1} = 2 \implies a_k = 5$
Let's test the chain starting with a_k+1 = 1$a_{k+1} = 1$:
- a_k = 2(1) + 1 = 3$a_k = 2(1) + 1 = 3$
- a_k-1 = 2(3) + 1 = 7$a_{k-1} = 2(3) + 1 = 7$
- a_k-2 = 2(7) + 1 = 15$a_{k-2} = 2(7) + 1 = 15$
- a_k-3 = 2(15) + 1 = 31$a_{k-3} = 2(15) + 1 = 31$
- a_k-4 = 2(31) + 1 = 63$a_{k-4} = 2(31) + 1 = 63$
- a_k-5 = 2(63) + 1 = 127$a_{k-5} = 2(63) + 1 = 127$ (but 127 notin mathrmA$127 \notin \mathrm{A}$!)
Thus, the longest chain within the set A$A$ has 6 elements:
\63, \, 31, \, 15, \, 7, \, 3, \, 1\$$\{63, \, 31, \, 15, \, 7, \, 3, \, 1\}$$
This chain corresponds to exactly 5 ordered pairs:
(63, 31), \, (31, 15), \, (15, 7), \, (7, 3), \, (3, 1)$$(63, 31), \, (31, 15), \, (15, 7), \, (7, 3), \, (3, 1)$$
So the maximum number of pairs in the sequence is k = 5$k = 5$.
### Step 2: Check alternative chains
If we start with a_k+1 = 2$a_{k+1} = 2$:
- a_k+1 = 2$a_{k+1} = 2$
- a_k = 5$a_k = 5$
- a_k-1 = 11$a_{k-1} = 11$
- a_k-2 = 23$a_{k-2} = 23$
- a_k-3 = 47$a_{k-3} = 47$
- a_k-4 = 95$a_{k-4} = 95$
- a_k-5 = 191 > 100$a_{k-5} = 191 > 100$
Again, the maximum number of pairs is k = 5$k = 5$. Thus, the largest integer k$k$ is 5.
### Pattern Recognition
Recursive scaling: Tracing exponential chains of the form x_n+1 = c x_n + d$x_{n+1} = c x_n + d$ shows that the elements grow very quickly. Calculating the limits of growth determines the maximum possible depth of the sequence.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 11 Mathematics: Relations and Functions
Q62
jee_main_2025_02_april_evening
Domain of a Function
If the domain of the function
f(x) = frac1sqrt10 + 3x - x^2 + frac1sqrtx + |x|$$f(x) = \frac{1}{\sqrt{10 + 3x - x^2}} + \frac{1}{\sqrt{x + |x|}}$$
is (a, b)$(a, b)$, then (1 + a)^2 + b^2$(1 + a)^2 + b^2$ is equal to:
Solution
### Related Formula
textFor frac1sqrtg(x) text to be defined, we require: g(x) > 0$$\text{For } \frac{1}{\sqrt{g(x)}} \text{ to be defined, we require: } g(x) > 0$$
### Core Logic
We find the domains of the two constituent terms separately and then find their intersection.
### Step 1: Find the domain of the first term
For the first term to be defined:
10 + 3x - x^2 > 0 implies x^2 - 3x - 10 < 0$$10 + 3x - x^2 > 0 \implies x^2 - 3x - 10 < 0$$
(x - 5)(x + 2) < 0 implies x in (-2, 5) quad text--- (1)$$(x - 5)(x + 2) < 0 \implies x \in (-2, 5) \quad \text{--- (1)}$$
### Step 2: Find the domain of the second term
For the second term to be defined:
x + |x| > 0$x + |x| > 0$
- If x ge 0$x \ge 0$: x + x = 2x > 0 implies x > 0$x + x = 2x > 0 \implies x > 0$.
- If x < 0$x < 0$: x - x = 0 ngtr 0$x - x = 0 \ngtr 0$.
Thus, the domain of the second term is:
x in (0, infty) quad text--- (2)$$x \in (0, \infty) \quad \text{--- (2)}$$
### Step 3: Find intersection and calculate the final expression
Intersecting domains (1) and (2):
x in (-2, 5) cap (0, infty) implies x in (0, 5)$$x \in (-2, 5) \cap (0, \infty) \implies x \in (0, 5)$$
Comparing this with (a, b)$(a, b)$ gives a = 0$a = 0$ and b = 5$b = 5$.
Now calculate the value:
(1 + a)^2 + b^2 = (1 + 0)^2 + 5^2 = 1 + 25 = 26$$(1 + a)^2 + b^2 = (1 + 0)^2 + 5^2 = 1 + 25 = 26$$
### Pattern Recognition
Modulus domain constraint: The function x + |x|$x + |x|$ is non-zero only for positive values of x$x$. This is a standard math trick that collapses complex domains down to x > 0$x > 0$ instantly.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 11 Mathematics: Relations and Functions