(1)若a‖b,则b=ka
即(cosa,2)=k(sina,1)=(ksina,k)
即k=2,cosa=ksina
tana=1/2
(2)a.b=(sina,1).(cosa,2)=sina*cosa+2=17/8
sin2a=1/4,cos2a=√[1-(1/4)^2]=√15/4
sin(2a+π/4)=sin2a*cos(π/4)+cos2a*sin(π/4)
=1/4*√2/2+√15/4*√2/2=(√2+√30)/8