sin2x=1/2,所以2x=30°或150°
则 x=15°或75°
sin15°
=sin(45°-30°）
=（1/2)√2 x(1/2)√3-(1/2)√2 x(1/2)
=(1/4)(√6 -√2)
sin75°
=sin(45°+30°）
=（1/2)√2 x(1/2)√3+(1/2)√2 x(1/2)
=(1/4)(√6 +√2)