It's part of a proportion.
The cross products in a proportion are equal.
example:
3/4 = 15/20
4x15 = 60
3x20 = 60
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cross: torque dot: work
The cross product can be said to be a measure of the 'perpendicularity' of the vectors in the product. Please see the link.
You flip it to a fraction
Normally you use sine theta with the cross product and cos theta with the vector product, so that the cross product of parallel vectors is zero while the dot product of vectors at right angles is zero.
You need to take the magnitude of the cross-product of two position vectors. For example, if you had points A, B, C, and D, you could take the cross product of AB and BC, and then take the magnitude of the resultant vector.