A force of 39 kg weight is acting at a
point P(−4, 2, 5) in the direction
12i−4j−3k. The moment of this force
about a line through the origin having
the direction of 2i−2j+k is
Show that the angle θ between two unit
vectors a_ ^ and b_ ^ is given by cosθ=a_ ^ •b_ ^ .
Hence, given that a_ ^ =i_ cosA+j_ sinA and
b_ ^ =i_ cosB−j_ sinB, prove that cos(A+B)=
cosAcosB−sinAsinB.