cho A(1; 2; -1), B(2; -1; 3), C(-4; 7; 5)

a) Đ

b) Đ

c) Đ

d) S

 

Ta có: = (2 – 1; −1 – 2; 3 – (−1)) = (1; −3; 4),

           = (−4 – 1; 7 – 2; 5 – (−1)) = (−5; 5; 6).

            AB =   = ,

            AC =   = .

Có: . = 1.(−5) + (−3).5 + 4.6 = 4.

cos = cos() = = = .