We need to show that the four points $A = -6i + 3j + 2k$, $B = 3i - 2j + 4k$, $C = 5i + 7j + 3k$, and $D = -13i + 17j - k$ are coplanar.

GeometryVectors3D GeometryCoplanar PointsScalar Triple ProductDeterminants
2025/6/15

1. Problem Description

We need to show that the four points A=6i+3j+2kA = -6i + 3j + 2k, B=3i2j+4kB = 3i - 2j + 4k, C=5i+7j+3kC = 5i + 7j + 3k, and D=13i+17jkD = -13i + 17j - k are coplanar.

2. Solution Steps

To show that four points are coplanar, we can show that the three vectors formed by these points are coplanar. Let's find the vectors ABAB, ACAC, and ADAD.
AB=BA=(3i2j+4k)(6i+3j+2k)=(3(6))i+(23)j+(42)k=9i5j+2kAB = B - A = (3i - 2j + 4k) - (-6i + 3j + 2k) = (3 - (-6))i + (-2 - 3)j + (4 - 2)k = 9i - 5j + 2k
AC=CA=(5i+7j+3k)(6i+3j+2k)=(5(6))i+(73)j+(32)k=11i+4j+kAC = C - A = (5i + 7j + 3k) - (-6i + 3j + 2k) = (5 - (-6))i + (7 - 3)j + (3 - 2)k = 11i + 4j + k
AD=DA=(13i+17jk)(6i+3j+2k)=(13(6))i+(173)j+(12)k=7i+14j3kAD = D - A = (-13i + 17j - k) - (-6i + 3j + 2k) = (-13 - (-6))i + (17 - 3)j + (-1 - 2)k = -7i + 14j - 3k
The points AA, BB, CC, and DD are coplanar if the scalar triple product of the vectors ABAB, ACAC, and ADAD is equal to zero. The scalar triple product is given by the determinant of the matrix formed by the components of the vectors.
Scalar triple product = AB(AC×AD)=95211417143AB \cdot (AC \times AD) = \begin{vmatrix} 9 & -5 & 2 \\ 11 & 4 & 1 \\ -7 & 14 & -3 \end{vmatrix}
=941143(5)11173+2114714= 9 \begin{vmatrix} 4 & 1 \\ 14 & -3 \end{vmatrix} - (-5) \begin{vmatrix} 11 & 1 \\ -7 & -3 \end{vmatrix} + 2 \begin{vmatrix} 11 & 4 \\ -7 & 14 \end{vmatrix}
=9(4(3)1(14))+5(11(3)1(7))+2(11(14)4(7))= 9(4(-3) - 1(14)) + 5(11(-3) - 1(-7)) + 2(11(14) - 4(-7))
=9(1214)+5(33+7)+2(154+28)= 9(-12 - 14) + 5(-33 + 7) + 2(154 + 28)
=9(26)+5(26)+2(182)= 9(-26) + 5(-26) + 2(182)
=234130+364= -234 - 130 + 364
=364+364= -364 + 364
=0= 0
Since the scalar triple product is zero, the vectors ABAB, ACAC, and ADAD are coplanar. Therefore, the points AA, BB, CC, and DD are coplanar.

3. Final Answer

The four points are coplanar.

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