（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 .你寫的這種方法我看不懂，可以換一種方法把它寫的詳細(xì)一些？sin2a=3√7/8,a∈[兀/4，兀/2],2a∈[兀/2，兀],cos2a<0,cos2a=-√[1-(sin2a)^2]=-1/8cos2a=1-2(sina)^2，2(sina)^2=1+1/8=9/8(sina)^2=9/16sina =3/4 --------------(sina)^2+(cosa)^2 = 1sin2a= 2sina * cosa （sin2a)^2=(3/8)^2 * 7=63/64 = 4sina * sina * cosa *cosa(sina)^2 * (cosa)^2 =63/256(sina)^2 * [1-(sina)^2 ]=63/256設(shè)(sina)^2=tt-t^2=63/256,(t-9/16)(t-7/16)=0(sina)^2 = 9/16 或7/16,a∈[四分之兀，二分之兀],sina ≥√2/2=√8/4,(sina)^2 ≥1/2=8/16.(sina)^2 = 9/16sina =3/4請(qǐng)問cos2a=1一2(sina)的平方怎么來的？cos2a=cosa *cosa-sina * sina=1 - 2(sina)^2我懂了，謝謝你。