Given vectors $a = (3, 3, 1)$, $b = (-2, -1, 0)$, and $c = (-2, -3, -1)$, we need to find the following: (a) $a \times b$ (b) $a \times (b + c)$ (c) $a \cdot (b \times c)$ (d) $a \times (b \times c)$

GeometryVectorsCross ProductDot ProductVector Algebra
2025/4/9

1. Problem Description

Given vectors a=(3,3,1)a = (3, 3, 1), b=(2,1,0)b = (-2, -1, 0), and c=(2,3,1)c = (-2, -3, -1), we need to find the following:
(a) a×ba \times b
(b) a×(b+c)a \times (b + c)
(c) a(b×c)a \cdot (b \times c)
(d) a×(b×c)a \times (b \times c)

2. Solution Steps

(a) a×ba \times b
a×b=ijk331210=i(301(1))j(301(2))+k(3(1)3(2))=i(0+1)j(0+2)+k(3+6)=i2j+3ka \times b = \begin{vmatrix} i & j & k \\ 3 & 3 & 1 \\ -2 & -1 & 0 \end{vmatrix} = i(3 \cdot 0 - 1 \cdot (-1)) - j(3 \cdot 0 - 1 \cdot (-2)) + k(3 \cdot (-1) - 3 \cdot (-2)) = i(0 + 1) - j(0 + 2) + k(-3 + 6) = i - 2j + 3k
So, a×b=(1,2,3)a \times b = (1, -2, 3).
(b) a×(b+c)a \times (b + c)
First, calculate b+cb + c:
b+c=(2,1,0)+(2,3,1)=(4,4,1)b + c = (-2, -1, 0) + (-2, -3, -1) = (-4, -4, -1).
Now, calculate a×(b+c)a \times (b + c):
a×(b+c)=ijk331441=i(3(1)1(4))j(3(1)1(4))+k(3(4)3(4))=i(3+4)j(3+4)+k(12+12)=ij+0ka \times (b + c) = \begin{vmatrix} i & j & k \\ 3 & 3 & 1 \\ -4 & -4 & -1 \end{vmatrix} = i(3 \cdot (-1) - 1 \cdot (-4)) - j(3 \cdot (-1) - 1 \cdot (-4)) + k(3 \cdot (-4) - 3 \cdot (-4)) = i(-3 + 4) - j(-3 + 4) + k(-12 + 12) = i - j + 0k
So, a×(b+c)=(1,1,0)a \times (b + c) = (1, -1, 0).
(c) a(b×c)a \cdot (b \times c)
First, calculate b×cb \times c:
b×c=ijk210231=i((1)(1)0(3))j((2)(1)0(2))+k((2)(3)(1)(2))=i(10)j(20)+k(62)=i2j+4kb \times c = \begin{vmatrix} i & j & k \\ -2 & -1 & 0 \\ -2 & -3 & -1 \end{vmatrix} = i((-1) \cdot (-1) - 0 \cdot (-3)) - j((-2) \cdot (-1) - 0 \cdot (-2)) + k((-2) \cdot (-3) - (-1) \cdot (-2)) = i(1 - 0) - j(2 - 0) + k(6 - 2) = i - 2j + 4k
So, b×c=(1,2,4)b \times c = (1, -2, 4).
Now, calculate a(b×c)a \cdot (b \times c):
a(b×c)=(3,3,1)(1,2,4)=(31)+(3(2))+(14)=36+4=1a \cdot (b \times c) = (3, 3, 1) \cdot (1, -2, 4) = (3 \cdot 1) + (3 \cdot (-2)) + (1 \cdot 4) = 3 - 6 + 4 = 1.
(d) a×(b×c)a \times (b \times c)
From part (c), we know that b×c=(1,2,4)b \times c = (1, -2, 4).
Now, calculate a×(b×c)a \times (b \times c):
a×(b×c)=ijk331124=i(341(2))j(3411)+k(3(2)31)=i(12+2)j(121)+k(63)=14i11j9ka \times (b \times c) = \begin{vmatrix} i & j & k \\ 3 & 3 & 1 \\ 1 & -2 & 4 \end{vmatrix} = i(3 \cdot 4 - 1 \cdot (-2)) - j(3 \cdot 4 - 1 \cdot 1) + k(3 \cdot (-2) - 3 \cdot 1) = i(12 + 2) - j(12 - 1) + k(-6 - 3) = 14i - 11j - 9k
So, a×(b×c)=(14,11,9)a \times (b \times c) = (14, -11, -9).

3. Final Answer

(a) a×b=(1,2,3)a \times b = (1, -2, 3)
(b) a×(b+c)=(1,1,0)a \times (b + c) = (1, -1, 0)
(c) a(b×c)=1a \cdot (b \times c) = 1
(d) a×(b×c)=(14,11,9)a \times (b \times c) = (14, -11, -9)

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