- A, and
- B, and
- C, and
- D, and
View written solutionFree
Correct answer: A, D
Let the given system be written as
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
We must find which option systems have a solution for every .
1. First find the set
A vector belongs to iff it lies in the column space of . So we first determine the consistency condition for the given system.
Row relation in
Observe the rows of :
Now,
which is not equal to .
Let us compute rank by elimination:
Take :
Take :
So,
For consistency, the last two equations in must agree:
Thus
Cross-multiplying:
Hence
So is a plane in .
2. Key idea for each option
If an option has coefficient matrix , then the system
has a solution for every iff
Since is a 2-dimensional plane, this means the column space of must contain exactly that plane (or all of if rank ).
Equivalently, if , then its consistency condition must be the same plane equation:
We now test each option.
3. Option A
System:
Coefficient matrix:
1&2&3\\ 0&4&5\\ 1&2&6 \end{bmatrix}.Compute determinant:
More directly by expansion along first row:
So is invertible. Therefore for every , the system has a unique solution. Hence certainly for every .
So A is correct.
4. Option B
System:
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
This gives
From coordinates:
So only first two are independent. Subtracting,
then
Thus one row relation is
Hence consistency requires the same relation on RHS:
But vectors in satisfy
which is a different plane. So not every will satisfy option B.
For example, take . Then
so . But
so option B is inconsistent.
Therefore B is incorrect.
5. Option C
System:
Coefficient matrix:
-1&2&-5\\ 2&-4&10\\ 1&-2&5 \end{bmatrix}.Observe:
So all rows are multiples of one row. Hence rank is . Thus consistency requires
This is a line in , not the whole plane . Therefore it certainly cannot hold for every .
For example, take again . But here we would need
which fails.
Hence C is incorrect.
6. Option D
System:
Coefficient matrix:
1&2&5\\ 2&0&3\\ 1&4&-5 \end{bmatrix}.Compute determinant:
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,
Thus is invertible, so the system has a unique solution for every , hence for every .
So D is correct.
7. Final conclusion
The systems that have at least one solution for each are:
This matches the stored correct answer.
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