已知sina=1/2,求 sin（π/4 α）sin（π/4-α）的值
问题描述：
已知sina=1/2,求 sin（π/4 α）sin（π/4-α）的值
答
根据积化和差公式：sin（π/4+ α）sin（π/4-α） =-[cos(π/4+ α+π/4-α)-cos(π/4+ α-π/4+α)]/2 =-[cosπ/2-cos2a]/2 =cos2a/2 =(1-2sina)/2 =(1-2×1/4)/2 =1/4 =1/2