tana=[2tan(a/2)]/[1-tan(a/2)^2]=20/21(2sin a+3cos a)/(3cos a-4 sin a)分子分母全除以cosa,则：(2sin a+3cos a)/(3cos a-4 sin a)=(2tana+3)/(3-4tana)=(2*20/21+3)/(3-4*20/21)=-103/17