cosx=-1/3 180°＜x＜270°
sinx=-2√2/3
180°＜x＜270°
360°＜2x＜540°
0°＜2x＜180°
sin2x
=2sinxcosx
=2*(-2√2/3 )*(-1/3)
=4√2/9
cos2x
=1-2cos²x
=1-2*(-1/3)²
=1-2/9
=7/9
tan2x
=sin2x/cos2x
=(4√2/9)/(7/9)
=4√2/7
cos x=1-2sin²x/2
-1/3=1-2sin²x/2
2sin²x/2=4/3
sin²x/2=2/3
sin²x/2=2/3
sinx/2=±√6/3
180°＜x＜270°
90°＜x/2＜135°
sinx/2>0
cosx/2