Yuxuan is a quantitative relationship, with Party B+Party C -2BCCOSA = Party A, and others with Party B -bcco sa+ Party C -BCCOSA = Party A..

B = B the square of B vector, BCCOSA = the dot product of vector C, and in the same way, it is deduced that B vector times B vector -c vector +c vector times C vector -b vector = the square of A, and the * * * factor has the square of B vector -c vector, and B vector -c vector = the sum of A vectors.

The square of the vector is equal to the square of the module.