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Solve a vector problem of high school mathematics college entrance examination!
M*N= root number 3sin(A+B)= root number 3sinC;;

1+cos(A+B)= 1-cosC;

Namely:

Root number 3sinc =1-cosc;

The solution is sinC=0 or sinC= root number 3/2; Obviously equal to zero impossible, does not constitute a triangle;

So sinC= root number 3/2, we can get: cosC= 1/2.

So C= 120 degrees.