tana=sina/cosa=2
sina=2cosa sin^2a+cos^2a=1 5cos^2a=1 cos^2a=1/5 sin^2a=4/5
2sin^2a-3sina*cosa-2cos^2a
=2sin^2a-6cos^2a-2cos^2a
=2sin^2a-8cos^2a
=8/5-8/5=0