Question

已知向量OA=(cosa,sina),OB=(3-cosa,4-sina),若向量OA‖OB

Answer 1

cosa(4-sina)-sina(3-cosa)=0

4cosa-sinacosa+sinacosa-3sina=0

4cosa=3sina

=4/3

cos2a=(1-tan^2a)/(1+tan^2a)=(1-16/9)/(1+16/9)=-7/25