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13. If [tex] \vec{A}=6 \vec{i}+5 \vec{j} [/tex] and [tex] \vec{B}=4 \vec{i}-3 \vec{j} [/tex], then [tex] (\vec{A}+\vec{B}) \cdot (\vec{A}-\vec{B}) [/tex] equals:

A. 18
B. 36
C. 72
D. 54


Sagot :

To find the value of [tex]\((\vec{A} + \vec{B}) \cdot (\vec{A} - \vec{B})\)[/tex], we proceed as follows:

1. Determine the components of [tex]\(\vec{A}\)[/tex] and [tex]\(\vec{B}\)[/tex]:
[tex]\[ \vec{A} = 6 \vec{i} + 5 \vec{j} \quad \text{and} \quad \vec{B} = 4 \vec{i} - 3 \vec{j} \][/tex]

2. Calculate [tex]\(\vec{A} + \vec{B}\)[/tex]:
[tex]\[ \vec{A} + \vec{B} = (6 \vec{i} + 5 \vec{j}) + (4 \vec{i} - 3 \vec{j}) = (6 + 4)\vec{i} + (5 - 3)\vec{j} = 10 \vec{i} + 2 \vec{j} \][/tex]

3. Calculate [tex]\(\vec{A} - \vec{B}\)[/tex]:
[tex]\[ \vec{A} - \vec{B} = (6 \vec{i} + 5 \vec{j}) - (4 \vec{i} - 3 \vec{j}) = (6 - 4)\vec{i} + (5 + 3)\vec{j} = 2 \vec{i} + 8 \vec{j} \][/tex]

4. Compute the dot product [tex]\((\vec{A} + \vec{B}) \cdot (\vec{A} - \vec{B})\)[/tex]:
[tex]\[ (10 \vec{i} + 2 \vec{j}) \cdot (2 \vec{i} + 8 \vec{j}) = (10 \cdot 2) + (2 \cdot 8) \][/tex]

5. Perform the multiplications:
[tex]\[ 10 \cdot 2 = 20 \][/tex]
[tex]\[ 2 \cdot 8 = 16 \][/tex]

6. Add the results:
[tex]\[ 20 + 16 = 36 \][/tex]

Therefore, the value of [tex]\((\vec{A} + \vec{B}) \cdot (\vec{A} - \vec{B})\)[/tex] is [tex]\(\boxed{36}\)[/tex].
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