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Find the vector, not with determinants, but by using properties of cross products

Math Jan 16, 2021

Find the vector, not with determinants, but by using properties of cross products. k times (i - 7j) Find the vector, not with determinants, but by using properties of cross products. (i + j) times (i - j) The figure shows a vector a the xy-plane and a vector b in the direction of k. Their lengths are Find |a times b|. Use the right-hand rule to decide whether the components of a times b are positive, negative x-component y-component z-component If a = (2, -1, 2) and b = (5, 2, 1), find the following. a times b = b times a =

Expert Solution

We recall the cross product property that

i \times i =0, \ j \times j =0, \ k \times k =0,

i \times j =k, \ j \times k =i, \ k \times i =j,

j \times i =-k, \ k \times j =-i, \ i \times k =-j

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Answer: 1.        We need to find k \times (i -7j)

   k \times (i -7j) = k\times i - 7(k\times j)

        = -j -7(-i)

=\ {\color{Blue} 7i-1j+ 0k}

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Answer 2.   Now we find (i+j)\times (i-j)

(i+j)\times (i-j) = i\times (i-j) +j\times (i-j)

(i+j)\times (i-j) = i \times i- i \times j+j \times i-j \times j

(i+j)\times (i-j) = 0- k+(-k)-0

(i+j)\times (i-j) = {\color{Blue} - 2k}

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Answer 4. Given a = \left \langle 2, -1, \ 2 \right \rangle and b = \left \langle 5, \ 2, \ 1 \right \rangle

a\times b =\left \langle 2, \ -1, \ 2 \right \rangle \times \left \langle 5, \ 2, \ 1 \right \rangle

 

Using distributive property and the properties of cross product

a\times b =4 k -2j +5k -1i +10j -4i  

a\times b ={\color{Blue} -5i +8j +9k}

Now we find b\times a

b\times a=\left \langle 5, \ 2, \ 1 \right \rangle \times \left \langle 2, \ -1, \ 2 \right \rangle

 

b\times a = -5k -10j -4k +4i +2j +1i

b\times a ={\color{Blue} 5i -8j -9k}

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