vector{АВ}=(4-1;7-3)=(3;4)
vector{CD}=(7-(-1);5-(-1))=(8;6)
|vector{АВ}|=sqrt(3^2+4^2)=5
|vector{CD}|=sqrt(8^2+6^2)=10
vector{АВ}•vector{CD}=3•8+4•6=24+24=48
cos(vector{АВ},vector{CD})=
=vector{АВ}•vector{CD}/|vector{АВ}|•|vector{CD}|=
=48/5•10=48/50=24/25
О т в е т. arccos(24/25)