由公式sina=y/√x²+y²=cos2／7π /(sin²2／7π+ cos²2／7π)=cos2／7π由诱导公式sina=cos(3π/2+a)∴cos(3π/2+a)=cos2／7π∴3π/2+a=2π/7+2kπa=2kπ-17π/14k=1时,正角a最小值就是11π/14...