Question

若a∈[四分之兀,二分之兀],sin2a=八分之三倍的根号七,则sina=?

Answer 1

（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 .