3sin^2α-sinαcosα-2cos^2α
=(3sin^2α-sinαcosα-2cos^2α)/1
=(3sin^2α-sinαcosα-2cos^2α)/(sin^2a+cos^2a) (上下同除以cos^2a)
=(3tan^2a-tana-2)/(tan^2a+1)
=12/5