Ta có:  AB→=−5+2;−1−4=−3;−5

 AC→=8+2;−2−4=10;−6

 BC→=8+5;−2+1=13;−1

Suy ra:  AB=AB→=−32+−52=34

 AC=AC→=102+−62=234

BC=BC→=132+−12=170

Ta có: AB→.AC→=−3.10+−5.−6=0  suy ra AB→  vuông góc với AC→  hay BAC^=90o .

Ta có: cosAC→,BC→=10.13+−6.−1102+62.132+−12=136234.170=25 .

Suy ra ACB^≈27o⇒ABC^=90o−ACB^≈63o .