若tana=1/3,则sin^3 a+2cos^ 3 a/sin^3 a-2cos^3 a

分子分母同除以cos^3a
(sin^3 a+2cos^ 3 a)/(sin^3 a-2cos^3 a)
=(sin³a/cos³a+2)/(sin³a/cos³a-2)
=(tan³a+2)/(tan³a-2)
=[(1/3)³+2]/[(1/3)³-2]
=(1+2*27)/(1-2*27)
=-55/53

=[sin A＋sinAcosA＋2cos A]/[sin A＋cos A]===分子分母同除以cos A===>==[tan A＋tanA＋2]/[1＋tan A]=7/5 sin A+sinAcosA+2cos