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Expert-verifiedTo 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.
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.
\(V = |{\rm{a}} \cdot ({\rm{b}} \times {\rm{c}})|(3)\)
\(P = ( - 2,1,0),Q = (2,3,2){\rm{ and R}} = (1,4, - 1)andS = (3,6,1)\)
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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