21. 如图,AAA,BBB是椭圆C:x2a2+y2b2=1(a>b>0)C:\frac{x^2}{a^2}+\frac{y^2}{b^2}=1(a>b>0)C:a2x2+b2y2=1(a>b>0)的左、右顶点,PPP是⊙O:x2+y2=a2\odot O:x^{2}+y^{2}=a^{2}⊙O:x2+y2=a2上不同于AAA,BBB的动点,线段PAPAPA与椭圆CCC交于点QQQ,若tan∠PBA=3tan∠QBA\tan \angle PBA=3\tan \angle QBAtan∠PBA=3tan∠QBA,则椭圆的离心率为((( )))