解A是第三象限角则2kπ+π/2＜A＜2kπ+3/2π,k属于Z即2/3kπ+π/6＜A/3＜2/3kπ+1/2π,k属于Z当k=3n时把k=3n代入2/3kπ+π/6＜A/3＜2/3kπ+1/2π得2nπ+π/6＜A/3＜2nπ+1/2π,n属于Z此时A/3是第一象限的角当k=3n+1...