sin2a=2sinacosa sin^2a+cos^2a=1
sin2a+sinacosa+2
=(2sinacosa+sinacosa+2sin^2a+2cos^2a)/(sin^2a+cos^2a ) 都除以cos^2a
=(3tana+2tan^2a+2)/(tan^2a+1)
∵tana=1/2
∴=(3*1/2+2*1/4+2)/(1/4+1)
=16/5