vector{BA}=(2-(-2); 5-0)=(4;5)
vector{BC}*vector{BA}=|vector{BC}|*|vector{BA}|*cos ∠ (vector{BC},vector{BA})
vector{BC}*vector{BA}=(-1)*4+4*5=16
|vector{BC}|=sqrt((-1)^2+4^2)=sqrt(17);
|vector{BA}|=sqrt(4^2+5^2)=sqrt(41);
cos ∠ (vector{BC},vector{BC})=16/(sqrt(17)*sqrt(41))