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Volume of parallelepiped determined by v...

Volume of parallelepiped determined by vectors `bara,barb` and `barc` is 5.
Then the volume of the parallelepiped determined by the vectors `3(bara +barb). (barb + barc)` and 2(`barc + bara)` is

A

`3(bara+barb),(barb+barc)` and `(barc+bara)` is 24

B

`2(baraxxbarb),3(barbxxbarc)` and `(barcxxbara)` is 24

C

`(bara-2barb),(barb-2barc)` and `(barc-2bara)` is 14

D

`3(bara-barb),(3barb-barc)` and `(3barc-bara)` is 52

Text Solution

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The correct Answer is:
To find the volume of the parallelepiped determined by the vectors \(3(\mathbf{a} + \mathbf{b})\), \((\mathbf{b} + \mathbf{c})\), and \(2(\mathbf{c} + \mathbf{a})\), we can use the properties of the scalar triple product (also known as the box product). ### Step-by-step Solution: 1. **Understand the Volume of Parallelepiped**: The volume \(V\) of a parallelepiped formed by vectors \(\mathbf{u}\), \(\mathbf{v}\), and \(\mathbf{w}\) can be calculated using the scalar triple product: \[ V = |\mathbf{u} \cdot (\mathbf{v} \times \mathbf{w})| \] 2. **Identify the Given Volume**: We are given that the volume of the parallelepiped determined by vectors \(\mathbf{a}\), \(\mathbf{b}\), and \(\mathbf{c}\) is 5: \[ V = |\mathbf{a} \cdot (\mathbf{b} \times \mathbf{c})| = 5 \] 3. **Express the New Vectors**: We need to find the volume of the parallelepiped formed by the vectors: \[ \mathbf{u} = 3(\mathbf{a} + \mathbf{b}), \quad \mathbf{v} = \mathbf{b} + \mathbf{c}, \quad \mathbf{w} = 2(\mathbf{c} + \mathbf{a}) \] 4. **Calculate the Scalar Triple Product**: The volume can be expressed as: \[ V' = |\mathbf{u} \cdot (\mathbf{v} \times \mathbf{w})| \] Substituting the expressions for \(\mathbf{u}\), \(\mathbf{v}\), and \(\mathbf{w}\): \[ V' = |3(\mathbf{a} + \mathbf{b}) \cdot ((\mathbf{b} + \mathbf{c}) \times (2(\mathbf{c} + \mathbf{a}))| \] 5. **Simplify the Cross Product**: First, calculate the cross product: \[ (\mathbf{b} + \mathbf{c}) \times (2(\mathbf{c} + \mathbf{a})) = 2[(\mathbf{b} + \mathbf{c}) \times \mathbf{c} + (\mathbf{b} + \mathbf{c}) \times \mathbf{a}] \] Since \(\mathbf{c} \times \mathbf{c} = \mathbf{0}\): \[ = 2[(\mathbf{b} \times \mathbf{a}) + (\mathbf{c} \times \mathbf{a})] \] 6. **Substitute Back into the Volume Expression**: Now substitute back into the volume expression: \[ V' = |3(\mathbf{a} + \mathbf{b}) \cdot (2[(\mathbf{b} \times \mathbf{a}) + (\mathbf{c} \times \mathbf{a})])| \] \[ = 6 |(\mathbf{a} + \mathbf{b}) \cdot [(\mathbf{b} \times \mathbf{a}) + (\mathbf{c} \times \mathbf{a})]| \] 7. **Expand the Dot Product**: \[ = 6 \left[ (\mathbf{a} \cdot (\mathbf{b} \times \mathbf{a})) + (\mathbf{b} \cdot (\mathbf{b} \times \mathbf{a})) + (\mathbf{a} \cdot (\mathbf{c} \times \mathbf{a})) + (\mathbf{b} \cdot (\mathbf{c} \times \mathbf{a})) \right] \] The terms \(\mathbf{a} \cdot (\mathbf{b} \times \mathbf{a})\) and \(\mathbf{b} \cdot (\mathbf{b} \times \mathbf{a})\) are zero because the dot product of a vector with itself or with a vector that is perpendicular to it is zero. 8. **Final Calculation**: Thus, we are left with: \[ = 6 |(\mathbf{b} \cdot (\mathbf{c} \times \mathbf{a}))| \] Since \(|\mathbf{a} \cdot (\mathbf{b} \times \mathbf{c})| = 5\), we have: \[ = 6 \times 5 = 30 \] ### Final Answer: The volume of the parallelepiped determined by the vectors \(3(\mathbf{a} + \mathbf{b})\), \((\mathbf{b} + \mathbf{c})\), and \(2(\mathbf{c} + \mathbf{a})\) is \(30\).
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