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Matrices and Determinants question

2018 · Shift 2 · Q21
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  5. /2018 · Shift 2 · Q21

Matrices and Determinants question

2018 · Shift 2 · Q21

JEE AdvancedMathematicsMatrices and DeterminantsMultiple correct+4 / −1
Let S be the set of all column matrices [b1b2b3]\left[ {\begin{matrix} {{b_1}} \\ {{b_2}} \\ {{b_3}} \\ \end{matrix} } \right]​b1​b2​b3​​​ such that b1,b2,b3∈R{b_1},{b_2},{b_3} \in Rb1​,b2​,b3​∈R and the system of equations (in real variables) −x+2y+5z=b12x−4y+3z=b2x−2y+2z=b3\begin{aligned} & - x + 2y + 5z = {b_1} \\ & 2x - 4y + 3z = {b_2} \\ & x - 2y + 2z = {b_3} \\\end{aligned}​−x+2y+5z=b1​2x−4y+3z=b2​x−2y+2z=b3​​ has at least one solution. Then, which of the following system(s) (in real variables) has (have) at least one solution for each [b1b2b3]\left[ {\begin{matrix} {{b_1}} \\ {{b_2}} \\ {{b_3}} \\ \end{matrix} } \right]​b1​b2​b3​​​ ∈\in∈ S?
  1. A
    x+2y+3z=b1x + 2y + 3z = {b_1}x+2y+3z=b1​,  4y+5z=b2\,4y + 5z = {b_2}4y+5z=b2​ and x+2y+6z=b3x + 2y + 6z = {b_3}x+2y+6z=b3​
  2. B
    x+y+3z=b1x + y + 3z = {b_1}x+y+3z=b1​, 5x+2y+6z=b25x + 2y + 6z = {b_2}5x+2y+6z=b2​ and −2x−y−3z=b3- 2x - y - 3z = {b_3}−2x−y−3z=b3​
  3. C
    −x+2y−5z=b1- x + 2y - 5z = {b_1}−x+2y−5z=b1​,  2x−4y+10z=b2\,2x - 4y + 10z = {b_2}2x−4y+10z=b2​ and x−2y+5z=b3x - 2y + 5z = {b_3}x−2y+5z=b3​
  4. D
    x+2y+5z=b1x + 2y + 5z = {b_1}x+2y+5z=b1​, 2x+3z=b22x + 3z = {b_2}2x+3z=b2​ and x+4y−5z=b3x + 4y - 5z = {b_3}x+4y−5z=b3​
View written solutionFree

Correct answer: A, D

Let the given system be written as

Ax=b,A\mathbf{x}=\mathbf{b},Ax=b, where

-1 & 2 & 5\\ 2 & -4 & 3\\ 1 & -2 & 2 \end{bmatrix}, \qquad \mathbf{b}=\begin{bmatrix}b_1\\b_2\\b_3\end{bmatrix}.

We are given that

S={b∈R3:Ax=b has at least one solution}.S=\left\{\mathbf{b}\in \mathbb R^3: A\mathbf{x}=\mathbf{b}\text{ has at least one solution}\right\}.S={b∈R3:Ax=b has at least one solution}.

We must find which option systems have a solution for every b∈S\mathbf{b}\in Sb∈S.


1. First find the set SSS

A vector b\mathbf{b}b belongs to SSS iff it lies in the column space of AAA. So we first determine the consistency condition for the given system.

Row relation in AAA

Observe the rows of AAA:

R1=(−1,2,5),R2=(2,−4,3),R3=(1,−2,2).R_1=(-1,2,5),\quad R_2=(2,-4,3),\quad R_3=(1,-2,2).R1​=(−1,2,5),R2​=(2,−4,3),R3​=(1,−2,2).

Now,

R1+R2=(−1+2, 2−4, 5+3)=(1,−2,8),R_1+R_2=(-1+2,\,2-4,\,5+3)=(1,-2,8),R1​+R2​=(−1+2,2−4,5+3)=(1,−2,8), which is not equal to R3R_3R3​.

Let us compute rank by elimination:

[−125b12−43b21−22b3]\begin{bmatrix} -1 & 2 & 5 & b_1\\ 2 & -4 & 3 & b_2\\ 1 & -2 & 2 & b_3 \end{bmatrix}​−121​2−4−2​532​b1​b2​b3​​​

Take R3→R3+R1R_3\to R_3+R_1R3​→R3​+R1​:

R3+R1=(0,0,7∣b1+b3).R_3+R_1=(0,0,7\mid b_1+b_3).R3​+R1​=(0,0,7∣b1​+b3​).

[−125b12−43b2007b1+b3]\begin{bmatrix} -1 & 2 & 5 & b_1\\ 2 & -4 & 3 & b_2\\ 0 & 0 & 7 & b_1+b_3 \end{bmatrix}​−120​2−40​537​b1​b2​b1​+b3​​​

Take R2→R2+2R1R_2\to R_2+2R_1R2​→R2​+2R1​:

R2+2R1=(0,0,13∣b2+2b1).R_2+2R_1=(0,0,13\mid b_2+2b_1).R2​+2R1​=(0,0,13∣b2​+2b1​).

So,

[−125b10013b2+2b1007b1+b3]\begin{bmatrix} -1 & 2 & 5 & b_1\\ 0 & 0 & 13 & b_2+2b_1\\ 0 & 0 & 7 & b_1+b_3 \end{bmatrix}​−100​200​5137​b1​b2​+2b1​b1​+b3​​​

For consistency, the last two equations in zzz must agree:

7z=b1+b3.\qquad 7z=b_1+b_3.7z=b1​+b3​.

Thus

b2+2b113=b1+b37.\frac{b_2+2b_1}{13}=\frac{b_1+b_3}{7}.13b2​+2b1​​=7b1​+b3​​.

Cross-multiplying:

7(b2+2b1)=13(b1+b3)7(b_2+2b_1)=13(b_1+b_3)7(b2​+2b1​)=13(b1​+b3​) 7b2+14b1=13b1+13b37b_2+14b_1=13b_1+13b_37b2​+14b1​=13b1​+13b3​ b1+7b2−13b3=0.b_1+7b_2-13b_3=0.b1​+7b2​−13b3​=0.

Hence

S={[b1b2b3]:b1+7b2−13b3=0}.S=\left\{\begin{bmatrix}b_1\\b_2\\b_3\end{bmatrix}: b_1+7b_2-13b_3=0\right\}.S=⎩⎨⎧​​b1​b2​b3​​​:b1​+7b2​−13b3​=0⎭⎬⎫​.

So SSS is a plane in R3\mathbb R^3R3.


2. Key idea for each option

If an option has coefficient matrix MMM, then the system

Mx=bM\mathbf{x}=\mathbf{b}Mx=b

has a solution for every b∈S\mathbf{b}\in Sb∈S iff

S⊆Col⁡(M).S\subseteq \operatorname{Col}(M).S⊆Col(M).

Since SSS is a 2-dimensional plane, this means the column space of MMM must contain exactly that plane (or all of R3\mathbb R^3R3 if rank 333).

Equivalently, if rank⁡(M)=2\operatorname{rank}(M)=2rank(M)=2, then its consistency condition must be the same plane equation:

b1+7b2−13b3=0.b_1+7b_2-13b_3=0.b1​+7b2​−13b3​=0.

We now test each option.


3. Option A

System:

x+2y+3z=b14y+5z=b2x+2y+6z=b3\begin{aligned} x+2y+3z&=b_1 \\ 4y+5z&=b_2 \\ x+2y+6z&=b_3 \end{aligned}x+2y+3z4y+5zx+2y+6z​=b1​=b2​=b3​​

Coefficient matrix:

1&2&3\\ 0&4&5\\ 1&2&6 \end{bmatrix}.

Compute determinant:

det⁡(MA)=1∣4526∣+3∣0412∣\det(M_A)= 1\begin{vmatrix}4&5\\2&6\end{vmatrix} +3\begin{vmatrix}0&4\\1&2\end{vmatrix}det(MA​)=1​42​56​​+3​01​42​​

More directly by expansion along first row:

det⁡(MA)=1(24−10)−2(0−5)+3(0−4)=14+10−12=12≠0.\det(M_A)=1(24-10)-2(0-5)+3(0-4) =14+10-12=12\neq 0.det(MA​)=1(24−10)−2(0−5)+3(0−4)=14+10−12=12=0.

So MAM_AMA​ is invertible. Therefore for every b∈R3\mathbf{b}\in\mathbb R^3b∈R3, the system has a unique solution. Hence certainly for every b∈S\mathbf{b}\in Sb∈S.

So A is correct.


4. Option B

System:

x+y+3z=b15x+2y+6z=b2−2x−y−3z=b3\begin{aligned} x+y+3z&=b_1 \\ 5x+2y+6z&=b_2 \\ -2x-y-3z&=b_3 \end{aligned}x+y+3z5x+2y+6z−2x−y−3z​=b1​=b2​=b3​​

Coefficient matrix:

1&1&3\\ 5&2&6\\ -2&-1&-3 \end{bmatrix}.

Find the consistency condition by finding a linear relation among rows. Suppose

αR1+βR2+γR3=0.\alpha R_1+\beta R_2+\gamma R_3=0.αR1​+βR2​+γR3​=0.

This gives

α(1,1,3)+β(5,2,6)+γ(−2,−1,−3)=(0,0,0).\alpha(1,1,3)+\beta(5,2,6)+\gamma(-2,-1,-3)=(0,0,0).α(1,1,3)+β(5,2,6)+γ(−2,−1,−3)=(0,0,0).

From coordinates:

α+5β−2γ=0\alpha+5\beta-2\gamma=0α+5β−2γ=0 α+2β−γ=0\alpha+2\beta-\gamma=0α+2β−γ=0 3α+6β−3γ=0  ⟺  α+2β−γ=0.3\alpha+6\beta-3\gamma=0 \iff \alpha+2\beta-\gamma=0.3α+6β−3γ=0⟺α+2β−γ=0.

So only first two are independent. Subtracting,

3β−γ=0  ⟹  γ=3β,3\beta-\gamma=0 \implies \gamma=3\beta,3β−γ=0⟹γ=3β, then

α+2β−3β=0  ⟹  α=β.\alpha+2\beta-3\beta=0 \implies \alpha=\beta.α+2β−3β=0⟹α=β.

Thus one row relation is

R1+R2+3R3=0.R_1+R_2+3R_3=0.R1​+R2​+3R3​=0.

Hence consistency requires the same relation on RHS:

b1+b2+3b3=0.b_1+b_2+3b_3=0.b1​+b2​+3b3​=0.

But vectors in SSS satisfy

b1+7b2−13b3=0,b_1+7b_2-13b_3=0,b1​+7b2​−13b3​=0, which is a different plane. So not every b∈S\mathbf{b}\in Sb∈S will satisfy option B.

For example, take b=(13,0,1)T\mathbf{b}=(13,0,1)^Tb=(13,0,1)T. Then

13+7(0)−13(1)=0,13+7(0)-13(1)=0,13+7(0)−13(1)=0, so b∈S\mathbf{b}\in Sb∈S. But

13+0+3≠0,13+0+3\neq 0,13+0+3=0, so option B is inconsistent.

Therefore B is incorrect.


5. Option C

System:

−x+2y−5z=b12x−4y+10z=b2x−2y+5z=b3\begin{aligned} -x+2y-5z&=b_1 \\ 2x-4y+10z&=b_2 \\ x-2y+5z&=b_3 \end{aligned}−x+2y−5z2x−4y+10zx−2y+5z​=b1​=b2​=b3​​

Coefficient matrix:

-1&2&-5\\ 2&-4&10\\ 1&-2&5 \end{bmatrix}.

Observe:

R2=−2R1,R3=−R1.R_2=-2R_1, \qquad R_3=-R_1.R2​=−2R1​,R3​=−R1​.

So all rows are multiples of one row. Hence rank is 111. Thus consistency requires

This is a line in R3\mathbb R^3R3, not the whole plane SSS. Therefore it certainly cannot hold for every b∈S\mathbf{b}\in Sb∈S.

For example, take again b=(13,0,1)T∈S\mathbf{b}=(13,0,1)^T\in Sb=(13,0,1)T∈S. But here we would need

b2=−2b1=−26,b_2=-2b_1=-26,b2​=−2b1​=−26, which fails.

Hence C is incorrect.


6. Option D

System:

x+2y+5z=b12x+3z=b2x+4y−5z=b3\begin{aligned} x+2y+5z&=b_1 \\ 2x+3z&=b_2 \\ x+4y-5z&=b_3 \end{aligned}x+2y+5z2x+3zx+4y−5z​=b1​=b2​=b3​​

Coefficient matrix:

1&2&5\\ 2&0&3\\ 1&4&-5 \end{bmatrix}.

Compute determinant:

det⁡(MD)=1∣034−5∣−2∣231−5∣+5∣2014∣.\det(M_D)=1\begin{vmatrix}0&3\\4&-5\end{vmatrix}-2\begin{vmatrix}2&3\\1&-5\end{vmatrix}+5\begin{vmatrix}2&0\\1&4\end{vmatrix}.det(MD​)=1​04​3−5​​−2​21​3−5​​+5​21​04​​.

Now,

egin{vmatrix}0&3\\4&-5\end{vmatrix}=0(-5)-12=-12, egin{vmatrix}2&3\\1&-5\end{vmatrix}=-10-3=-13, egin{vmatrix}2&0\\1&4\end{vmatrix}=8.

So,

det⁡(MD)=−12−2(−13)+5(8)=−12+26+40=54≠0.\det(M_D)=-12-2(-13)+5(8)=-12+26+40=54\neq 0.det(MD​)=−12−2(−13)+5(8)=−12+26+40=54=0.

Thus MDM_DMD​ is invertible, so the system has a unique solution for every b∈R3\mathbf{b}\in\mathbb R^3b∈R3, hence for every b∈S\mathbf{b}\in Sb∈S.

So D is correct.


7. Final conclusion

The systems that have at least one solution for each b∈S\mathbf{b}\in Sb∈S are:

A,D.\boxed{A, D}.A,D​.

This matches the stored correct answer.

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