(1)判别式=16(sina)^2-4atana=0,得sina=0或16sina-4a/cosa=0,所以16sinacosa-4a=0
所以8sin2a=4a,sin2a=a/2,因为-1<=sin2a<=1,所以-1<=a/2<=1,所以-2<=a<=2
(2)sin2a=a/2=7/8,因为sin2a=-cos(π/2+2a)=-cos2(π/4+a)=-[1-2sin(π/4+a)^2]=-1+2sin(π/4+a)^2
所以7/8=-1+2sin(π/4+a)^2,所以sin(π/4+a)^2=15/16,所以sin(π/4+a)=正负根号下15/4