Question Details

Let  S = { a + b 2 : a , b Z } , T 1 = { ( 1 + 2 ) n : n N } and T 2 = { ( 1 + 2 ) n : n N } . Then which of the following statements is (are) TRUE?

Options

A

Z T 1 T 2 S

B

T 1 ( 0 , 1 2024 ) = ϕ , where ϕ denotes the empty set.

C

T 2 ( 2024 , ) ϕ

D

For any given  a , b Z , cos ( π ( a + b 2 ) ) + i sin ( π ( a + b 2 ) ) Z  if and only if b = 0, where  i = 1

Show Answer

Correct Answer :

Option A

Z T 1 T 2 S

Option C

T 2 ( 2024 , ) ϕ

Option D

For any given  a , b Z , cos ( π ( a + b 2 ) ) + i sin ( π ( a + b 2 ) ) Z  if and only if b = 0, where  i = 1

ℤ ∪ T₁ ∪ T₂ ⊂ S; T₂ ∩ (2024, ∞) ≠ ϕ; and for any given a,b ∈ ℤ, cos(π(a + b√2)) + i sin(π(a + b√2)) ∈ ℤ if and only if b = 0, where i = √-1.

Solution :

To determine which of the statements are true, let us analyze each of them step-by-step.

First, recall the definitions of the sets given in the problem statement:
S = { a + b 2 : a , b Z }
T 1 = { ( 1 + 2 ) n : n N }
T 2 = { ( 1 + 2 ) n : n N }

Step 1: Analyze the statement ZT1T2S
Let us check if each of the subsets Z, T1, and T2 is contained in S.
1. For any integer kZ, we can write k=k+02. Since k,0Z, we have kS. Thus, ZS.
2. Note that the set S is closed under addition, subtraction, and multiplication because it is the ring of integers of the quadratic field Q(2). Specifically, if x,yS, then xyS.
Since 1+2S (with a=1,b=1), any positive integer power (1+2)n must also belong to S by mathematical induction. Hence, T1S.
3. Similarly, since 1+2S (with a=1,b=1), any positive integer power (1+2)n must also belong to S. Hence, T2S.
Combining these results, we get ZT1T2S. Therefore, this statement is TRUE.

Step 2: Analyze the statement T1(0,12024)=ϕ
Let us analyze the base of the exponent in T1:
Since 1.414<2<1.415, we have:
0 < 1 + 2 < 0.415 < 1
For a base r=21 where 0<r<1, the sequence xn=rn is strictly decreasing and approaches 0 as n.
Thus, for a sufficiently large n, we can always find rn<12024.
Therefore, the intersection T1(0,12024) is not empty. This statement is FALSE.

Step 3: Analyze the statement T2(2024,)ϕ
Let us analyze the base of the exponent in T2:
Since 1+22.414>1, the sequence yn=(1+2)n is strictly increasing and diverges to as n.
Thus, we can definitely find a positive integer n such that (1+2)n>2024 (for instance, 2.49>2500>2024).
Therefore, the intersection T2(2024,) is non-empty. This statement is TRUE.

Step 4: Analyze the statement: For any given a,bZ, cos(π(a+b2))+isin(π(a+b2))Z if and only if b=0
Using Euler's formula, let:
z = cos ( π ( a + b 2 ) ) + i sin ( π ( a + b 2 ) ) = e i π ( a + b 2 )
If zZ, then z must be a real number. For z to be real, the imaginary part must be zero:
sin ( π ( a + b 2 ) ) = 0
This occurs if and only if the argument is an integer multiple of π:
π ( a + b 2 ) = k π for some  k Z
Dividing by π:
a + b 2 = k
Rearranging the equation:
b 2 = k a
Since a,kZ, the right-hand side ka is an integer.
If b0, then 2=kab, which implies that 2 is rational. This is a contradiction since 2 is irrational.
Thus, we must have b=0.
Conversely, if b=0, then:
z = cos ( π a ) + i sin ( π a ) = ( 1 ) a
Since aZ, (1)a{1,1}Z.
Hence, zZ if and only if b=0. This statement is TRUE.

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