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Essential Calculus: Early Transcendentals
Found in: Page 565
Essential Calculus: Early Transcendentals

Essential Calculus: Early Transcendentals

Book edition 2nd
Author(s) James Stewart
Pages 830 pages
ISBN 9781133112280

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Short Answer

To find: The volume of the parallelepiped determined with adjacent edges PQ, PR and PS.

The volume of the parallelepiped with the adjacent edges PQ, PR and PS is \(16\) cubic units.

See the step by step solution

Step by Step Solution

Step 1: Concept of parallelepiped with the help of Formula’s

Consider two three-dimensional vectors such as,

\(\begin{array}{l}a = \left\langle {{a_1},{a_2},{a_3}} \right\rangle \\{\rm{b}} = \left\langle {{b_1},{b_2},{b_3}} \right\rangle \end{array}\)

Write the expression for cross product between \(a\) and \({\rm{b}}\) vectors.

\(a \times b = \left| {\begin{array}{*{20}{c}}{\rm{i}}&{\rm{j}}&{\rm{k}}\\{{a_1}}&{{a_2}}&{{a_3}}\\{{b_1}}&{{b_2}}&{{b_3}}\end{array}} \right|(1)\)

Write the expression for dot product between \(a\) and \({\rm{b}}\) vectors.

\(a \cdot b = {a_1}{b_1} + {a_2}{b_2} + {a_3}{b_3}(2)\)

Write the expression to calculate the parallelepiped's volume.

\(V = |{\rm{a}} \cdot ({\rm{b}} \times {\rm{c}})|(3)\)

Step 2: Volume of parallelepiped by calculating through Formula.

\(P = ( - 2,1,0),Q = (2,3,2){\rm{ and R}} = (1,4, - 1)andS = (3,6,1)\)

Consider the adjacent edges as \({\rm{a}} = \overrightarrow {PQ} ,\;{\rm{b}} = \overrightarrow {PR} \) and \({\rm{c}} = \overrightarrow {PS} \)

Find \({\rm{a}} = \overrightarrow {PQ} \).

\(\begin{array}{l}\overrightarrow {PQ} = Q(2,3,2) - P( - 2,1,0)\\\overrightarrow {PQ} = \langle (2 + 2),(3 - 1),(2 - 0)\rangle \\\overrightarrow {PQ} = \langle 4,2,2\rangle \end{array}\)

Find \({\rm{b}} = \overrightarrow {PR} \).

\(\begin{array}{l}\overrightarrow {PR} = R(1,4, - 1) - P( - 2,1,0)\\\overrightarrow {PR} = \langle (1 + 2),(4 - 1),( - 1 - 0)\rangle \\\overrightarrow {PR} = \langle 3,3, - 1\rangle \end{array}\)

Find \({\rm{c}} = \overrightarrow {PS} \).

\(\begin{array}{l}\overrightarrow {PS} = S(3,6,1) - P( - 2,1,0)\\\overrightarrow {PS} = \langle (3 + 2),(6 - 1),(1 - 0)\rangle \\\overrightarrow {PS} = \langle 5,5,1\rangle \end{array}\)

Modify equation (\(1\)).

\(b \times c = \left| {\begin{array}{*{20}{c}}{\rm{i}}&{\rm{j}}&{\rm{k}}\\{{b_1}}&{{b_2}}&{{b_3}}\\{{c_1}}&{{c_2}}&{{c_3}}\end{array}} \right|\)

We can substitute \(3\) for \({b_1},3\) for \({b_2}, - 1\) for \({b_3},5\) for \({c_1},1\) for \({c_2}\) and \(1\) for \({c_3}\),

\(\begin{array}{l}b \times c = \left| {\begin{array}{*{20}{c}}{\rm{i}}&{\rm{j}}&{\rm{k}}\\3&3&{ - 1}\\5&5&1\end{array}} \right|\\b \times c = \left| {\begin{array}{*{20}{l}}3&{ - 1}\\5&1\end{array}} \right|{\rm{i}} - \left| {\begin{array}{*{20}{l}}3&{ - 1}\\5&1\end{array}} \right|{\rm{j}} + \left| {\begin{array}{*{20}{l}}3&3\\5&5\end{array}} \right|{\rm{k}}\\b \times c = (3 + 5){\rm{i}} - (3 + 5){\rm{j}} + (15 - 15){\rm{k}}\\b \times c = 8{\rm{i - }}8{\rm{j}} - 0{\rm{k}}\end{array}\)S

Modify equation \(\left( 2 \right)\)

\(a \cdot (b \times c) = {a_1}\left( {{b_1} \times {c_1}} \right) + {a_2}\left( {{b_2} \times {c_2}} \right) + {a_3}\left( {{b_3} \times {c_3}} \right)\)

We are substituting \(4\) for \({a_1},2\) for \({a_2},2\) for \({a_3},8\) for \(\left( {{b_1} \times {c_1}} \right), - 8\) for \(\left( {{b_2} \times {c_2}} \right)\) and \(0\) for \(\left( {{b_3} \times {c_3}} \right)\),

\(\begin{array}{l}a \cdot (b \times c) = 4(8) + 2( - 8) + 2(0)\\a \cdot (b \times c) = 32 - 16 + 0\\a \cdot (b \times c) = 16\end{array}\)

We are substituting \(16\) for \(a \cdot (b \times c)\) in equation \((3)\)

\(\begin{array}{l}V = |16|\\V = 16\end{array}\)

Thus, the volume of the parallelepiped determined by the vectors \(a\), \(b\) and \(c\) is \(16\) cubic units.

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