Given vectors
a ⃗ = − 5 i ^ + j ^ − 3 k ^ , b ⃗ = i ^ + 2 j ^ − 4 k ^ \vec a = -5\hat i + \hat j - 3\hat k, \qquad \vec b = \hat i + 2\hat j - 4\hat k a = − 5 i ^ + j ^ − 3 k ^ , b = i ^ + 2 j ^ − 4 k ^
We need
c ⃗ = ( ( ( ( a ⃗ × b ⃗ ) × i ^ ) × i ^ ) × i ^ ) \vec c = ((((\vec a \times \vec b) \times \hat i) \times \hat i) \times \hat i) c = (((( a × b ) × i ^ ) × i ^ ) × i ^ )
and then compute
c ⃗ ⋅ ( − i ^ + j ^ + k ^ ) . \vec c \cdot (-\hat i + \hat j + \hat k). c ⋅ ( − i ^ + j ^ + k ^ ) .
Find a ⃗ × b ⃗ \vec a \times \vec b a × b
Using the determinant:
a ⃗ × b ⃗ = ∣ i ^ j ^ k ^ − 5 1 − 3 1 2 − 4 ∣ \vec a \times \vec b=
\begin{vmatrix}
\hat i & \hat j & \hat k \\
-5 & 1 & -3 \\
1 & 2 & -4
\end{vmatrix} a × b = i ^ − 5 1 j ^ 1 2 k ^ − 3 − 4
= i ^ ( 1 ⋅ ( − 4 ) − ( − 3 ) ⋅ 2 ) − j ^ ( ( − 5 ) ⋅ ( − 4 ) − ( − 3 ) ⋅ 1 ) + k ^ ( ( − 5 ) ⋅ 2 − 1 ⋅ 1 ) = \hat i\,(1\cdot(-4)-(-3)\cdot2)
- \hat j\,((-5)\cdot(-4)-(-3)\cdot1)
+ \hat k\,((-5)\cdot2-1\cdot1) = i ^ ( 1 ⋅ ( − 4 ) − ( − 3 ) ⋅ 2 ) − j ^ (( − 5 ) ⋅ ( − 4 ) − ( − 3 ) ⋅ 1 ) + k ^ (( − 5 ) ⋅ 2 − 1 ⋅ 1 )
= i ^ ( − 4 + 6 ) − j ^ ( 20 + 3 ) + k ^ ( − 10 − 1 ) = \hat i(-4+6) - \hat j(20+3) + \hat k(-10-1) = i ^ ( − 4 + 6 ) − j ^ ( 20 + 3 ) + k ^ ( − 10 − 1 )
= 2 i ^ − 23 j ^ − 11 k ^ = 2\hat i - 23\hat j - 11\hat k = 2 i ^ − 23 j ^ − 11 k ^
So,
p ⃗ = a ⃗ × b ⃗ = 2 i ^ − 23 j ^ − 11 k ^ . \vec p = \vec a \times \vec b = 2\hat i - 23\hat j - 11\hat k. p = a × b = 2 i ^ − 23 j ^ − 11 k ^ .
Compute successive cross products with i ^ \hat i i ^
Let
p ⃗ = ( 2 , − 23 , − 11 ) . \vec p = (2,-23,-11). p = ( 2 , − 23 , − 11 ) .
For any vector ( x , y , z ) (x,y,z) ( x , y , z ) ,
( x , y , z ) × i ^ = ( 0 , z , − y ) . (x,y,z) \times \hat i = (0,z,-y). ( x , y , z ) × i ^ = ( 0 , z , − y ) .
First cross product
p ⃗ × i ^ = ( 0 , − 11 , 23 ) \vec p \times \hat i = (0,-11,23) p × i ^ = ( 0 , − 11 , 23 )
So,
p ⃗ 1 = − 11 j ^ + 23 k ^ . \vec p_1 = -11\hat j + 23\hat k. p 1 = − 11 j ^ + 23 k ^ .
Second cross product
p ⃗ 1 × i ^ = ( 0 , 23 , 11 ) \vec p_1 \times \hat i = (0,23,11) p 1 × i ^ = ( 0 , 23 , 11 )
So,
p ⃗ 2 = 23 j ^ + 11 k ^ . \vec p_2 = 23\hat j + 11\hat k. p 2 = 23 j ^ + 11 k ^ .
Third cross product
p ⃗ 2 × i ^ = ( 0 , 11 , − 23 ) \vec p_2 \times \hat i = (0,11,-23) p 2 × i ^ = ( 0 , 11 , − 23 )
Hence,
c ⃗ = 11 j ^ − 23 k ^ . \vec c = 11\hat j - 23\hat k. c = 11 j ^ − 23 k ^ .
Now compute the dot product
c ⃗ ⋅ ( − i ^ + j ^ + k ^ ) = ( 0 , 11 , − 23 ) ⋅ ( − 1 , 1 , 1 ) \vec c \cdot (-\hat i + \hat j + \hat k)
= (0,11,-23)\cdot(-1,1,1) c ⋅ ( − i ^ + j ^ + k ^ ) = ( 0 , 11 , − 23 ) ⋅ ( − 1 , 1 , 1 )
= 0 ⋅ ( − 1 ) + 11 ⋅ 1 + ( − 23 ) ⋅ 1 = 0\cdot(-1) + 11\cdot1 + (-23)\cdot1 = 0 ⋅ ( − 1 ) + 11 ⋅ 1 + ( − 23 ) ⋅ 1
= 11 − 23 = − 12 = 11 - 23 = -12 = 11 − 23 = − 12
Compare with options
The value is
− 12 \boxed{-12} − 12
So the correct option is:
A .