1原式＝sin50(1+tan60*tan10)=sin50(1+sin60cos10/cos60cos10)
=sin50*cos(60-10)/cos60cos10=sin50cos50/0.5cos10=sin100/cos10
=cos(90-100)/cos10=1
2原式＝(2cos0.5acos0.5a-2sin0.5acos0.5a)/(2sin0.5asin0.5a-2sin0.5acos0.5a)=2cos0.5a(cos0.5a-sin0.5a)/2sin0.5a(sin0.5a-cos0.5a)
=-cot0.5a
3和第二道一样用2倍角公式展开,答案-tan0.5a
4用正弦定理得到等式2rsun2c+1=2rsin3c=2rsinc-1和sincsinc+cosccosc=1连理解三元方程,把cosc负根舍弃,（一时想不出好方法,这个给LZ参考一下,希望有所启发）