法一sin80°＝1−cos280°＝1−cos2(−80°)＝1−k2，所以tan100°=-tan80°=−sin80°cos80°＝−1−k2k．：法二cos（-80°）=k⇒cos（80°）=k，tan100°＝sin1000cos100o＝sin(1800−800)cos(1800−800)＝sin80o...