x是钝角,sinx=1/3,求cos2x-sin2x/cosx^2

x是钝角,sinx=1/3,求cos2x-sin2x/cosx^2
x是钝角,sinx=1/3,求cos2x-sin2x/cosx^2

x是钝角,sinx=1/3,求cos2x-sin2x/cosx^2
x是钝角,sinx=1/3,cosx＝-2√2/3
（cos2x-sin2x）/cosx^2
=(cos^2x-sin^2x-2sinxcosx)/cos^2x
=（8/9-1/9+2*1/3*2√2/3)/(8/9)
=(8-1+4√2)/8
=(5+4√2)/8