∵0＜a＜π/2,∴π/6＜a＋π/6＜π/2＋π/6＜π,又cos（a＋π/6）＝4/5＞0,∴a＋π/6为锐角,∴sin（a＋π/6）＝√｛1－［cos（a＋π/6）］^2｝＝√（1－16/25）＝3/5.∴sin（2a＋π/12）＝sin［（2a＋π/3）－π/3＋...