2cos^2 (a) - 1 = cos(2a)
根据万能公式：
sin(2a)=2tana/[1+tan^2(a)]=(2/3)/(1+1/9)=3/5
cos(2a)=[1-tan^2(a)]/[1+tan^2(a)]= 4/5
1/2 sin(2a) + con^2(a)
=1/2sin(2a) + 1/2[cos(2a) +1]
=3/10 + 1/2(9/5)
=6/5