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JEE Main 202325 JanuaryMorning Shift
Exam
JEE Main
Year
2023
Date
25 January
Shift
Morning Shift
Subject
Mathematics
Chapter
Determinants

JEE Main 2023 · 25 January · Morning Shift · Mathematics · Determinants

Let S _ 1 and S _ 2 be respectively the sets of all a ∈ R - \ 0\...

Let S1andS2{S}_{1} and {S}_{2} be respectively the sets of all a∈R−{0}a \in R - \left\{0\right\} for which the system of linear equations
ax + 2ay - 3az = 1</p><p>(2a+1)x+(2a+3)y+(a+1)z=2</p><p>\left(2a + 1\right)x + \left(2a + 3\right)y + \left(a + 1\right)z = 2

\left(3a + 5\right)x+(a+5)x + \left(a + 5\right)y + \left(a + 2\right)z=3z = 3

has unique solution and infinitely many solutions. Then

Options

  1. An(S1)=2n\left({S}_{1}\right) = 2 and S2{S}_{2} is an infinite set
  2. BS1{S}_{1} is an infinite set an n(S2)=2n\left({S}_{2}\right) = 2
  3. CS1=ϕ{S}_{1} = ϕ and S2=R−{0}{S}_{2} = \mathbb{R} - \left\{0\right\}
  4. DS1=R−{0}{S}_{1} = \mathbb{R} - \left\{0\right\} and S2=ϕ{S}_{2} = ϕ

Correct answer

D. S1=R−{0}{S}_{1} = \mathbb{R} - \left\{0\right\} and S2=ϕ{S}_{2} = ϕ

Step-by-step solution

Given,

S1{S}_{1} and S2{S}_{2} be respectively the sets of all a∈R−{0}a \in R - \left\{0\right\} for which the system of linear equations


ax+2ay−3az=1.........(1)ax + 2ay - 3az = 1 . . . . . . . . . \left(1\right)

\left(2a + 1\right)x + \left(2a + 3\right)y + \left(a + 1\right)z = 2 . . . . \left(2\right)</p><p>(3a+5)x+(a+5)y+(a+2)z=3.....(3)</p><p>\left(3a + 5\right)x + \left(a + 5\right)y + \left(a + 2\right)z = 3 . . . . . \left(3\right)

Now from above equations finding \Delta = \left|\begin{cases}a & 2a & - 3a \\ 2a + 1 & 2a + 3 & a + 1 \\ 3a + 5 & a + 5 & a + 2\end{cases}\right|

=a(15a2+31a+36)=0⇒a=0= a\left(15{a}^{2} + 31a + 36\right) = 0 ⇒ a = 0

As 15a2+31a+3615{a}^{2} + 31a + 36 cannot be zero as 312−4×15×36<0{31}^{2} - 4 \times 15 \times 36 < 0

So, Δ≠0\Delta \ne 0 for all a∈R−{0}a \in R - \left\{0\right\}

Hence, S1=R−{0} and S2=ϕ{S}_{1} = R - \left\{0\right\} \text{ and } {S}_{2} = ϕ

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