sinacosa
=1/2*sin2a
=1/2*2tana/(1+tan²a)
=tana/(1+tan²a)
=2/(1+2²)
=2/5
2sin²a-3sinacosa-4cos²a
=2(1-cos²a)-3sinacosa-4cos²a
=2-2cos²a-3sinacosa-4cos²a
=2-6cos²a-3sinacosa
=2-6(1+cos2a)/2-3/2*sin2a
=2-3(1+cos2a)-3/2*sin2a
=2-3-3cos2a-3/2*sin2a
=-1-3*(1-tan²a)/(1+tan²a)-3/2*2/5
=-1-3*(1-2²)/(1+2²)-3/5
=-1+9/5-3/5
=1/5