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Amir encoded a secret phrase using matrix multiplication. Using [tex]$A=1, B=2, C=3$[/tex], and so on, he multiplied the clear text code for each letter by the matrix
[tex]\[ C=\begin{bmatrix}-3 & 2 \\ -4 & 3\end{bmatrix} \][/tex]
to get a matrix that represents the encoded text. The matrix representing the encoded text is
[tex]\[ \begin{bmatrix}
10 & -26 & -49 & 9 & 3 & -13 & -50 \\
16 & -33 & -63 & 16 & 9 & -10 & -65
\end{bmatrix}. \][/tex]

What is the secret phrase? Determine the location of spaces after you decode the text.

A. THE BEAN IS GREEN

B. BLUE IS THE GLOVE

C. GREEN IS THE BEAN

D. THE GLOVE IS BLUE


Sagot :

To find the secret phrase encoded by Amir, we need to decode the text using the provided matrices. Here is the step-by-step solution:

### Step 1: Write Down the Given Matrices
We have two matrices:
1. The encoding matrix [tex]\( C \)[/tex]:
[tex]\[ C = \begin{pmatrix} -3 & 2 \\ -4 & 3 \end{pmatrix} \][/tex]

2. The encoded text matrix:
[tex]\[ \begin{pmatrix} 10 & -26 & -49 & 9 & 3 & -13 & -50 \\ 16 & -33 & -63 & 16 & 9 & -10 & -65 \end{pmatrix} \][/tex]

### Step 2: Find the Inverse of the Encoding Matrix [tex]\( C \)[/tex]
To decode the matrix, we need to find the inverse of [tex]\( C \)[/tex]:

The inverse of a [tex]\( 2 \times 2 \)[/tex] matrix [tex]\(\begin{pmatrix} a & b \\ c & d \end{pmatrix}\)[/tex] is given by:
[tex]\[ \frac{1}{ad - bc} \begin{pmatrix} d & -b \\ -c & a \end{pmatrix} \][/tex]

Plugging in the values for [tex]\( C \)[/tex]:
[tex]\[ \text{Det}(C) = (-3)(3) - (2)(-4) = -9 + 8 = -1 \][/tex]

So, the inverse [tex]\( C^{-1} \)[/tex] is:
[tex]\[ C^{-1} = \frac{1}{-1} \begin{pmatrix} 3 & -2 \\ 4 & -3 \end{pmatrix} = \begin{pmatrix} -3 & 2 \\ -4 & 3 \end{pmatrix} \][/tex]

### Step 3: Multiply the Encoded Matrix by the Inverse of [tex]\( C \)[/tex]
Let [tex]\( E \)[/tex] be the encoded matrix:
[tex]\[ E = \begin{pmatrix} 10 & -26 & -49 & 9 & 3 & -13 & -50 \\ 16 & -33 & -63 & 16 & 9 & -10 & -65 \end{pmatrix} \][/tex]

We need to calculate [tex]\( C^{-1} \times E \)[/tex]:
[tex]\[ C^{-1} \times E = \begin{pmatrix} -3 & 2 \\ -4 & 3 \end{pmatrix} \times \begin{pmatrix} 10 & -26 & -49 & 9 & 3 & -13 & -50 \\ 16 & -33 & -63 & 16 & 9 & -10 & -65 \end{pmatrix} \][/tex]

### Step 4: Decode the Numerical Matrix into Letters
Once we obtain the resulting matrix after the multiplication, we map the numerical values back to letters according to [tex]\( A=1, B=2, C=3, \ldots, Z=26 \)[/tex], and assume [tex]\( 0 \)[/tex] represents a space.

### Step 5: Check with Possible Phrases
Given the possible phrases are:
1. THE BEAN IS GREEN
2. BLUE IS THE GLOVE
3. GREEN IS THE BEAN
4. THE GLOVE IS BLUE

The decoded numerical values will map to one of these phrases.

### Conclusion
After decoding the matrix and analyzing the numerical values, you will find that the correct secret phrase is:

[tex]\[ \boxed{\text{BLUE IS THE GLOVE}} \][/tex]
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