则sina=?若a∈[四分之兀?
若a∈[四分之兀,二分之兀],sin2a=八分之三倍的根号七,则sina=?
(sin2a)^2=(3/8)^2 *7=63/64=4sina sina cosa cosa sina sina *(1-sina sina) = 63/64/4=63/256 sina sina=t, t-t^2=63/256, (t-9/16)(t-7/16)=0 sina sina=9/16 或7/16, a∈[四分之兀,二分之兀],sina >0 sina =3/4 或√7/4 .
答:tan a/2 = sin a/2 / cos a/2 = 1/2 2 * sin a/2 = cos a/2, 2 sin a = cos^2 a/...详情>>
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