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It comes from the Law of Cosines.

* * * * *

For any two vectors A and B, the projection of A onto B, that is, the component of A along B, is ab.cos(x) where x is the angle between the two vectors. By symmetry, this is also the projectoin of B onto A.

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Q: Why you are using cosine function in dot product of two vectors?
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How do you do rotation in math?

A vector rotation in math is done on a coordinate plane.2D vectors can be rotated using the cross and dot product.3D vectors are rotated using matrix based quaternion math.

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Cross product is not difine in two space why?

When performing the cross product of two vectors (vector A and vector B), one of the properites of the resultant vector C is that it is perpendicular to both vectors A & B. In two dimensional space, this is not possible, because the resultant vector will be perpendicular to the plane that A & B reside in. Using the (i,j,k) unit vector notation, you could add a 0*k to each vector when doing the cross product, and the resultant vector will have zeros for the i & jcomponents, and only have k components.Two vectors define a plane, and their cross product is always a vector along the normal to that plane, so the three vectors cannot lie in a 2D space which is a plane.

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If you know the angle's sine, cosine, or tangent, enter it into the calculator and press <inverse> sine, cosine, or tangent. On MS Calc, in Scientific Mode, using Degrees, enter 0.5, then check Inv and the press sin. You should get 30 degrees. The other functions work similarly.