(1+cosa-sina)/(1-cosa-sina)+(1-cosa-sina)/(1+cosa-sina)=2sina/(cos^2a-1) 这部分没问题是对的 但后面错了 最后=2sina/(1-sin^2a-1)=-2/sina
tan20'+4sin20'=(sin20+2sin40)/cos20=
(sin20+sin40+sin40)/cos20=
(2sin30cos10+sin40)/cos20=
(cos10+sin40)/cos20=
(sin80+sin40)/cos20=2sin60cos20/cos20=2sin60=根号3
secx-tanx=1/cosx-sinx/cosx=(1-sinx)/cosx
=(coxx/2-sinx/2)^2/(cosx/2)^2-(sinx/2)^2
=(coxx/2-sinx/2)/（coxx/2-sinx/2)
tan(π/4 - x/2)=(1-tanx/2)/(1+tanx/2)
=(coxx/2-sinx/2)/（coxx/2-sinx/2)
∴secx - tanx = tan(π/4 - x/2)
sinA+sinB)(sinA+sinB)＝1+2sinAsinB=9／25
sinAsinB=-8/25
(cosA+cosB)(cosA+cosB)=1+2cosAcosB=16/25
cosAcosB=-9/50
cos(A-B)=cosAcosB+sinAsinB=-1/2
SIN(A+B)+COS(A+B)=或者＝根号2倍的cos(A+B-45度）＝根号2倍的[cos(A+B)cos45度+sin(A+B)sin45]
=1/2[cos(A+B)+sin(A+B)]
=2分之根号2倍的cos0
=2分之根号2