1.
2sin(2x)=sina+cosa (1)
(sinx)^2=sina*cosa (2)
2.
(1)^2-4*(2)==>
4[4(cosx)^2-1]*(sinx)^2=[sina-cosa]^2
==>
(cosx)^2≥1/4.
3.
(1)^2-2*(2)==>
2[8(cosx)^2-1]*(sinx)^2=1
==>
16(cosx)^4-18(cosx)^2+3=0
==>
(cosx)^2=[9±√33]/16
根据2.得
(cosx)^2=[9+√33]/16
==>
cos2x=2(cosx)^2-1=[1+√33]/8