diketahui segitiga ABC dengan A(3,1,-1) B(2,3,1) dan C (-1,2,-4).besar sudut BAC adalah

jawab

AB = {(3-2), (1- 3), (-1-1)
AB = (1, - 2, - 2)
|AB| = √(1² +(-2)²+(-2)² = √9 = 3

AC = {(3+1),(1-2), (-1+4)}
AC = (4, -1, 3)
|AC| = √(16+1+9) = √26

(AB).(AC) = { 1(4) -2(-1) -2(3) } = 4 + 2 - 6 = 0

cos BAC = (AB)(AC) / |AB| |AC|
cos BAC  = 0/(3√26) = 0
<BAC = 90