sin a-2cos a=0 ,则
tana=2,
则sina=2/根号5,-2/根号5
cosa=1/根号5,-1/根号5
则cos^2=1/5
4sin^2 a-3sin a x cos a-5cos^2 a
=1/cos^2 a(4tan^2a-3tana-5)
=1/cos^2 a(4*2^2-3*2-5)
=5/cos^2
=5*5
=25